The 3D N-S equations are numerically solved, while NND2M scheme is used for the discretization of the equations, aimed to simulate the dynamic flow about the process in which booster takes apart from upper stage. The effect that the jet of the counter-thrust rocket motor exerts on the axial force on the base of upper stage body is analyzed. The results of the simulation show a law that counter-thrust rocket jet has a great effect on the upper stage body while the distance between them is short, which produces a big axial force doing harm to the flight stability of upper stage.