The dynamic overset method and Eulerian approach were applied to solve the unsteady NS function and 6DOF movement function, and to numerically simulate the separation processes of KSR-III rocket’s core and to booster with and without additional forces. Major factors that affect the separation processes were also analyzed. It shows that the numerical results agree with the experimental results, indicating that the dynamic overset assembly method is able to correctly predict the complex flow between the rocket’s core and booster. Furthermore, based on the results of numerical simulations, separation parameters and rules of the booster under different conditions are also obtained.