I have a very simple CGI webserver running using python CGIHTTPServer class.
This class spawns and executes a cgi-php script.
If the webpage sends POST request, how can I access the POST request data in the php script?
When you say your Python process "spawns and executes" a cgi-php script, I believe what you mean is "it calls my PHP script by executing the PHP CLI executable, passing it the name of my script."
Using the PHP CLI executable, HTTP-specific superglobals and environment values will not be set automatically.  You would have to read in all HTTP request headers and GET/POST data in your Python server process, and then set them in the environment used by your PHP script.
The whole experiment sounds interesting, but this is what mod_php does already.
Accessing POST request data from CGIHTTPServer (python)