I eventually solved this problem today.
Acctually it is not the problem of platform IO.This problem happened because there is a encrpytion software on my computer which encrypted the .c & .cpp file but did not considered VSCODE as encrypted software. So VScode can not read the .c/.cpp file in default library and the intellisense do not work.
But it is really interesting that platform IO can build well .i am puzzled at that ,and have long way to go in this development tool.
thanks again. 
PS.It takes two work days after I report the problem to my boss to get an after-sale service engineer of the encryption software to help me. 