Head-Up Display (HUD) onboard civil aircrafts is a crucial safety system.Because of its high complexity and other airborne systems combined with it,the traditional system-safety-assessment method has difficulty in meeting the requirements of a quantitative safety analysis.Therefore,it′s necessary to develop a Model-Based Safety-Assessment (MBSA) method.On the basis of clearly defining the principles of the probability model check and the high-level system-modeling specifications,we studied the method for hierarchical modeling of the probability model of the HUD system,built the probability model of the HUD system,described the quantitative safety properties of the system,and carried out automatic probability model checks.The conclusion of the quantitative safety analysis was obtained,which can improve the efficiency of the safety analysis and the accuracy of the calculating results.