Aiming at the impact force caused by piston movement of automatic rifle during underwater shooting, research on piston movement law. Separately establish piston movement model during underwater shooting and calculation model of water flow to piston impact velocity, analyze piston movement condition, ascertain water flow velocity under projectile push. The research results show that: for the 5.8 mm amphibious rifle, the piston will move only when the velocity of the projectile to the air guiding position is greater than 360.25 m/s. The results of this paper can be used for the dynamic analysis of the automatic mechanism of pneumatic rifle under the condition of underwater shooting.