Who can tell me what c51 do in asm when I call OS_CREATE_TASK?

Why not use simulator+disassembler and find out?

I have been used simulator+disassembler
,but I don't understand.

It's generally best to treat the Operating System as a "Black Box"
Why do you need to delve into its internal workings?

"I don't understand"

That's why it's best to just treat the OS as a "Black Box!!" ;-)

See: http://www.keil.com/forum/msgpage.asp?MsgID=4372

I want to learn how c51 deal with
"?RTX_RAMTOP"、"?RTX_CURRENTTASK"、
"?RTX_TASKSP"、"?RTX_TASKTOP"、"?RTX_CURRENTTASK"、"?RTX_TASKENTRY"
and so on.

Read the manual?

Thank you a lot!