|Thanks a lot for both of you!|
I checked cpu and psu fans and found that the psu fan was running well, but the cpu fan stopped running occasionally during the power on but the computer did not shut down right away. However, after I cleaned fans, this phenomenon did not happen again.
The computer shuts down in a second and the front lights are off too.
This morning, I tried to run the computer again. The running time before it shutting down is random. The first time of running lasted the longest, and then the running time became shorter and shorter. It restarted by itself contunually for 5 times before it was completely off.