In order to solve the complicated calculation problem in the process of separation and ejection of shroud separation device, a simulation calculation method of the whole separation process was proposed. Based on the working principle of the small rocket engine, the mathematical model of the separation device was established through the law of gas generation. The shearing process of the broken screw, the movement process of the shell and the side throwing process of the shroud separation device were analyzed respectively. The interior ballistic, the axial and radial separation speeds were simulated and calculated. The simulation results showed that the method has reference value for product design, which could save research and development cost and development cycle.