In order to analyze stability of missile in terminal guidance and its impact on accuracy, condition of trajectory stability based on proportional navigation law was studied Firstly. The design and analysis methods on trajectory stability based on proportional navigation law were proposed. Then, error source of helicopter-borne antitank missile system was modeled and the analysis system on accuracy was designed by using precision analysis methods. Finally, relying on the error source model which has high precision and accuracy and the analysis system on accuracy designed in this paper, digital simulation tests were completed and the accuracy analysis result with high confidence and the assessment result of terminal guidance control system were obtained.