I ask the person who installed this server.
it was not one time installation.
their steps on the installation is:
(1) install engine, not include management studio and agent service.
(2) install SP 2 .
(3) reinstall engine and management studio and agent service without uninstall previous components.
(4) install sp2 again.
I slao made a test of the maintenance plan.
I write a simple T-SQL script on management studio, it runs good,
then I create a maintenance plan task to run only this T-SQL script,
result is the same with the full back up task,doesn't work.
So I think the problem is conflict of the original installation.