Abstract: This paper deals with the recoiling movement of an anti-air gun. In the paper, the structure composed of recoiling mass and anti-recoil device is considered as a nonlinear system and then simulated by the nonlinear modal synthesis technique, in which the stiff mechanical parts are trested as the linear sub-systems, and the springs, frictional forces and rigid contacts as the nonlinear links between the sub-systems. The results are given, which show good agreements with those from tests. Also the force on gun frame by the anti-recoil device is calculated. And it is pointed out that the gun frame is excited not by the impulse force but by an oscillating force.