Abstract： To combine the advantages of each submarines and missiles in combat, the piston pulling mode which was generally used in landbased missiles was ported to underwater missiles launched by submarines. A numerical simulation model coupled calculation of unsteady flow and motion was established to carry out piston pulling launched missile. The process of the missile launched underwater and air while ejecting out of launching tube was calculated by the given initial conditions. The evolution of missile launching flow and motion parameters was given. The simulation results indicate that there is no enough time for the sea water to fill up the space caused by the motion of the missile during the underwater launching process. The unfilled space comes into cavitation, which increases the pressure drag of the missile, thus reducing the velocity of the missile and increasing the time for ejecting out of launching tube. The cavitation around the missile tail raises the uncertainty of the missile hydrodynamic characteristic, and has an adverse effects on the design and controlling of internal ballistics. The impact load on the tail and the bottom of the tube of collapse and perish by the cavitation will pose higher requirements for structure protection of those places.