Based on system theory process analysis （STPA）， a safety development method for high level autonomous driving decision systems was proposed. The proposed method was applied to the prototype of an urban autonomous driving decision system. 70 Unsafe Control Actions （UCAs） for the entire system were found through safety analysis. Focusing on 3 function states of the system， 10 reasons for UCAs and 9 safety strategies were put forward. One strategy got implemented and validated through model-in-the-loop simulations. Test results show， the safety strategy is effective and the proposed method can enhance the safety of autonomous driving decision systems.