After a few days trying I finally found the reason is that I used to set the hand parent to something and after that I didn't reparent the hand to the figure itself!

Maybe it's just that the constraints are the same color as the background, but even when I went deliberately trying to see them by moving the hip up that the feet lift well off the floor, I don't.

Anyway, the alternatives to using IK are either manually adjusting the positions of the arms/legs to keep the hands/feet in the correct place, which is tedious and not super precise, or you could try Richard60's suggestion of using Constraints. I have not played with that myself.

But IK generally will do what you want. I would just suggest saving before you turn it on/off, and then check things are ok. Set keyframes to avoid them getting messed up by the switch. The whole thing about targets is largely moot if you aren't moving the body so far that the hands/feet are getting dragged away from their starting point.

The IK target is only visible while you hold and move say the hip and pull the body away from the ground.  As soon as you release the hip then the IK targets disappear.  Which is why they are only slightly useful.  In that you can see where the target is when moving but not always and if the foot/hand is still at the target location then it is hidden, and you never see it.  It would be really nice to have the targets visible all the time, except that you would not see them if they were covered

Eh..this may be a stupid question,but may I ask what should I do when I want to make an animation,such as a man doing push-ups without using IK?

Try it the other way around.  That is apply IK FIRST then the pose.  When IK is off the hand is not moved rather the shoulder and elbow move the hand.  The problem is that the hand has a known location, and IK is trying to apply that location to the hand and the hand is no longer at that location because it was moved by the rest of the arm, so when IK is turned on the hand is trying to move to where it thinks it should be.

For the Key Frames I would change them to Linear since it is a controlled known movement.  The problem with the default is that the Spline try's to go through all the points and that messes with the movements before and after you place a new key frame.  I will post a couple of examples when I am at home and can get to them.  Or you can search for posts that I have made, and you will see some of the issues.

IK Test Jump by Richard60

above is a link to a video that shows how to use IK and constraints to try and do what you want.  I had a thread or an answer to a thread about how to do IK.  I will try and look that up later also.  The thing is that I would put in a prop (ball) that you can be the constraint for the hand.  Put the ball where the default hand is and constrain and turn on IK.  Now slowly move the ball prop to where you want it to touch.  Adjust the Wrist of the hand along the way to keep it in the proper real-life position.  Do this by adjusting the constraint object and not the hand. This will be somewhere along the timeline, and you will be working backwards to bring the hand to the starting position either by moving the ball with the constraint on OR by dialing down the constraint till the hand is free at 0.  Now when you dial the constraint up the hand reaches for it.  ONCE you have the movement exactly the way you want it then you will bake in all the movements so that you can work without the IK.  

IK is one of those things that you use all the time or none of the time.  However, you can make small sections using IK to get the movement you want and then bake it in and turn off IK.  Tha way you don't break the rule of IK On or Off both not both.

IK is one of those things that is best to use or not use.  If you use it start from the beginning and move the hand which will move the rest of the arm.  What it appears you have done is posed the figure then turned on IK and the hand is still set to it's default position and it jumped back there and since it is an impossible move the rest of the arm distorts.  If you set the figure to default pose and then tried to move the hand to in front of the chest without changing the bend angles of the wrist you end up with weird angles.  IK for the feet are easier since the foot almost always faces forward. The hand will change direction simply by moving it.  IK for the arms is best used when the hands are constrained to another object, such as a steering wheel.  That way the hands move a bit and the arms follow without as much distortion.  

The question is why do you need to have IK on the hands on?

IF you are doing an animation and want to use IK for the hands it is best if you set the Keyframes to Linear as that will prevent overshoot that occurs when there are several keys and they are not symmetrical. 

When you drop an OBJ file you should see an import options dialog. If that's not happening something is definitely not right. This is what you should see ...

It seems more likely the OBJ is either too big or too small to see in the preview. First try setting the scale to ...

"100% " and "of standard figure" Set the option "Centered" and "Place on floor". Uncheck all the other options.

You should see the object appear after clicking [OK].

If that allows you to see the imported OBJ it's now just a matter of figuring it's real scale. Poser's scale is very small compared to most other apps. So try scaling to 10% or 1% "of original size". Once you find the right scale just import all the objects at an appropriate scale.

For example Blender scale is 0.382 compared to Poser scale.

You can drag and drop them from Explorer. It will import each one. They will be imported as separate objects. To my knowledge, there's no way you can get them to import as a single object if that's what you're looking for. You will either have to combine them in a modeler first or import them all into Poser and then export them all at once which will create one obj that you can then import as one item.

another option is to use 2 balls. Have one be a control ball. Set it to be invisible in retracing and in camera so it doesn't render. Parent the ball you want to scale and the hand to that control ball.

Did you unparent the ball before resizing it?