Firstly, the supercavitating vehicle was simulated by a cylindrical thin shell which was loaded with a axial dynamic load, and the dynamic stability differential equations and the unstable regions of structure were deduced. Secondly, the safety margin equations of dynamic buckling were given and linearized by limit step length iteration method when the randomness of axial load was taken into account, and step-by-step searching method was proposed in search of those effective safety margin equations. Finally, the reliability index of dynamic buckling was calculated with step-by-step equivalent plane method. Through numerical results, the influence of the change of load frequency, velocity and the coefficient of load rate to the dynamic buckling reliability were discussed. The calculate result give a theory foundation to how to choose the safety range of load frequency, velocity and the coefficient of load rate.