Yang Zhou says that there can be infinity python.exe in your computer. Maybe some install numpy while some not.
Some other interpreters on my computer: 
C:\Users\think\PycharmProjects\Test\venv\Scripts\python.exe       Even if I reinstall my operating system, the files in C:\Users\think\PycharmProjects do not need to be stored since this in an empty project.
E:\OneDrive - ustc.edu\PycharmProjects\Test\venv\Scripts
I have set C:\Users\think\Anaconda3\python.exe as the default interpreter. This interpreter has installed numpy. How can we set this? pycharm -> File -> Default settings
pycharm -> File -> settings -> show all -> existing environment
One interesting fact is that if I name a directory as _numpy and "import numpy as np" will have error. This error can be solved by changing the name of the directory from _numpy to _numpy1.