Abstract: Base on classical interior ballistics theory, kinetics model of piston dispersing for high-low pressure combustion chamber was established. Numerical calculation code was compiled by four-order Runge-Kutta method. A cluster warhead was calculated to obtain the pressure of high-low pressure combustion chamber, the velocity and acceleration of bullet. Changing rules of pressure,velocity and acceleration were acquired by analyzing the effect of fumarole area, powder mass and volume of combustion chamber for dispersing process. The calculated results are in agreement with actual changing rule, which has reference and guidance on the piston constructional design for cluster munitions.