Abstract: The influence of ejection depth and air filled in the launch tube on a tube-exit cavity was researched by numerical simulation methods during projectile ejected underwater. The ejection process was simulated based on a homogeneous multiphase flow, ideal gas state equation, homogeneous equilibrium flow cavitation model, and RNG k-ε equation combined with the dynamic mesh. The shape characteristics and inner structure of a tube-exit cavity was studied by conducted simulation on a typical underwater projectile shape model. Four types of ejection depth was simulated, and the variation of the elasticity coefficient of gas, when the tube-exit cavity closed, was also analyzed and validated with the theoretical analysis result given in the literature. The numerical results show that the even pressure gas moves with the projectile and forms a ventilated cavity on the surface of the projectile. The elasticity coefficient of the gas has great influence on the leakage speed and close point of the cavity. The deeper of the ejection, the earlier cavity closed with the same ejecting speed.