In order to study the motion law of ballistic missile and decoy， the flight program of ballistic missile in the active phase is designed in three steps. According to the force condition of ballistic missile in different motion stages， fully considering the factors such as atmospheric resistance and perturbation force， the full ballistic motion model including the active phase， free flight phase and reentry phase of ballistic missile is constructed in the geocentric inertial coordinate system， and the decoy release process of ballistic missile is modeled， and the motion equations of ballistic missile warhead and bait are established. The model is used to simulate the ballistic missile with four decoys and two-stage rocket boost， and the variation laws of the flight height， velocity， inclination and acceleration of the ballistic missile warhead and decoy with time are analyzed. The simulation results show that the model can fully and accurately reflect the motion characteristics of ballistic missile in several different motion stages.