To solve the problem of overload-recoil when launching the high muzzle velocity grenade by grenade machine gun system (GMGS), a new effective technology of reducing the recoil force using laval nozzle device with double chambers is presented, and the device is designed and applied to the GMGS. Based on unsteady one-dimensional gas dynamic theory, the interior ballistic duration and after-effect period gas dynamics equations of the recoil device were derived. The rationality of the built mathematical model was validated by contrasting the test date and simulation results. The gas pressure of back chamber, front chamber, gas operated device and nozzle chamber was calculated and the variation laws were analyzed. The bore pressure and muzzle velocity of laval nozzle device with double chambers and conventional weapons were compared. The distribution of gas parameter in nozzle with time and the distribution of gas parameter in nozzle with position were selectively analyzed. The influence of launching performance of laval nozzle with double chambers were discussed. The results show that the GMGS can achieve low recoil launching by setting reasonable parameters of the laval nozzle device with double chambers, while the projectile velocity is unaffected.