I find there is no defocus node in 2.77a. So how to do DOF in blender2.77a. Thank.

From the Blender 2.77 manual

Depth of Field

Thanks for stepping in here, keppel.

Just to add to Keppel's answer above, my apologies if this is too basic and you knew this already but you need to be in the compositor - your screenshot shows a material node, not a compositing one,so the defocus node will not be available since it is a post-render effect. Click the icon that looks like two pictures where I drew an arrow below.

You can do it "in camera" as well by using the settings in the camera (see the box I drew on your screenshot below) by adjusting the f-stop there. This will add DOF blurring at render time (but will also slow your render down by a large amount and possibly add a lot of noise unless you let it go for a much higher number of samples).

Thanks.I find that I have forgot to check"compositing".