Combined with the analysis of hit probability of an actual semi-active laser guided air-to-ground helicopter-borne missile weapon system, the missile system simulation software was established based on the analysis of the weapon system's error model. Missile hit error was counted utilizing Monte Carlo method, and the missile hit probability of all range was computed. The simulation results agree gree will with the experiments. The paper gives exact conclusion for semi-active laser guided air-to-ground missile weapon system performance evaluation.