For the influence of tail cavity on hydrodynamic damping force, a method was proposed by combing the rotating reference frames and cavity multiphase flow simulation techniques and solving the Reynolds averaged NS equations. Numerical research indicates that the unbalance of vehicle tail part pressure distribution has been relieved by tail cavity, which causes the reduction of hydrodynamic damping force. As the cavity expands, the decreasing rate trends tend to be slower. The tail cavity also changes the trend of damping force, which varies with the attack angle. Research reveals the necessary consideration of the effect of tail cavity on underwater vehicle hydrodynamic damping force design.