Inverse simulation is important in validating the known flying state. In the process of establishing equations, quaternion is used to set up six degree-of-freedom(6-DOF) flight model in order to avoid singularities，the flight simulation based on quaternion is also performed. The general method of inverse simulation is introduced; the means of identifying key parameters in the inverse simulation is discussed. The 3-DOF model trajectory acquired by avoiding a missile is inversely simulated to get the control rule for producing the initial trajectory, the inversely simulated trajectory is gained by running the 6-DOF model with the obtained control rules. The rationality of the initial optimal trajectory is verified by comparing the inversely simulated trajectory with the initial trajectory.