I found the way to solve this problem. This page & this page can explain such phenomenon. I am using .net472 and the HttpCliet.Post send two packets, one for header and the other one for the body. But somehow the target machine cannot handle the seperated post packets correctly. When it receives the first packet it responses and will not perform the posted data as expected. I cannot change the .net version so I solved this by platform invoking another dll like wininet.dll.