In order to study the effect of the piston reset motion on the flow of propellant gas in a piston-controlled side spray gun, the propulsion process of the gun was modeled and simulated based on the one-dimensional two-phase flow interior ballistic theory. First, by considering the reciprocating motion of the piston-spring system that controls the opening and closing of the rear spray channel, a mathematical model of the gun propulsion process was established. It combines the gas-solid two-phase flow in the barrel, the fluid-solid coupling between the piston and the gas in the piston cavity, and the transient gas flow in the exhaust pipe. The flow field coupling between the barrel and the piston cavity, and the flow field coupling between the piston cavity and the exhaust pipe were modeled, respectively, and the solution procedure was displayed. The MacCormack scheme and the Runge-Kutta method were used in the simulations, and the accuracy of the numerical method was validated by the published data. Next, the propagation law of the rarefaction wave in the barrel during the firing cycle was gained. The projectile velocity and the pressure at the projectile base and the breech were presented. Then, the distributions of the pressure, the gas velocity, and the solid velocity in the barrel of the piston-controlled side spray gun were compared with those in the traditional gun. Finally, the effects of the piston reset motion on the propellant gas flow and the recoil reduction efficiency were analyzed. The results show that compared with the situation ignoring the piston reset motion, when the muzzle velocity is reduced by 1.52%, the recoil reduction efficiency of the case considering the reset motion drops from 38.86% to 32.88% because the piston closes the rear spray channel during the reset process. Therefore, the reset motion of the piston cannot be ignored in the numerical simulation on the gun firing process.