In order to analyze the influence factor of PGR dispersion, the influence factor was determined by analyzing force of main rocket body and engine, i. e. reverse gas jet and separating speed error. Shooting test was simulated by Monte Carlo after choosing different separation points. The result of simulation shows the separating speed error only effects on range dispersion, which is worse with the increase of range and fire angle. Reverse gas jet has effect on both range dispersion and lateral dispersion. Lateral dispersion is worse with range and fire angle increase. Range dispersion is worse with range increase and better with fire angle increase. Trajectory parameter and air parameter at different separation point has no effect on dispersion.