Terrain Awareness and Warning System (TAWS) is designed to prevent the Controlled Flight Into Terrain (CFIT) accidents and enhance the flight safety.Based on the alert principle of Mode 3 the models of nominal trajectories and avoidance trajectories were established for the aircraft losing altitude.Two statistical parameters of incident rate and un-alerting incident rate were accumulated for a large number of flight events to evaluate the performance of the thresholds.A method based on system operating characteristic(SOC) was presented to get the alert envelope for losing altitude.The simulation results are identical to the practical envelope which show the correctness and effectiveness of our models.