What is the desktop environment of the physical display?
What is the desktop environment that you are seeing when you connect?
Assuming you have installed the free version, the free version of NoMachine will start its own virtual display server if it cannot find one, such as when a Linux machine is headless (no monitor attached). See more details about this here: https://kb.nomachine.com/AR10K00700
Meanwhile, when I shut down the computer, it is shown that a new user with same name is added.
What do you mean?
some Python GUI lib can not work on the new virtual display.
What do you mean?
Submitting screenshots would be useful to illustrate what the problem is.