I accidentally remove trash icon again in dolphin. This is the second one.
I accidentally click on remove trash while I want to empty trash. The old one I just delete the .kde folder in /home/user to restore it. But it erased the mail setting in kmail too. Fortunately I have a backup in other folder.
Today I discover a good news in http://forum.kde.org/viewtopic.php?f=66&t=87237
and there is a guy who tell us about what to do.

edit ~/.kde4/share/apps/kfileplaces/bookmarks.xml
and add this line

Now start dolphin, and the trash icon come back.