in npt process can not keep constant pressure

I think you should not worry about the pressure variation, it is a
normal thing. Look at the size of you simulation box, it is probably
not varying too much. I've never simulated a system big enough to have
small fluctuations on the pressure, therefore I don't know how big or
even if it is possible. You should look at some molecular dynamics
book to understand this better, or even the thermostat/barostat
original papers, it is always informative to do so.

Rodrigo