Abstract: In order to improve the real-time performance and reliability of trajectory identification based on GPS position, a trajectory parameters estimation algorithm by Kalman filtering is put forward, using GPS position data as observations and performing the optimal prediction by fourth Runge-Kutta numerical integration for solving projectile trajectory nonlinear differential equation. Improvement method is taken into account in case of failure or gross error of GPS receiver. Semi-physical simulation is used by Spirent GPS satellite signal simulator and C/A code receiver. The results show that the error of trajectory parameters estimation is approximately 30 percent to 40 percent of GPS raw measurement when the receiver works properly. Further, trajectory parameters estimation could be close to true value even if GPS receiver is abnormal.