To satisfy the safety requirement of civil aircraft engine, it is required that the risk of top level engine failure event shall be controlled to an acceptable level. According to the risk control requirements of top event, the expected probability of top level event shall be controlled below the acceptable value. Based on the predicted probability requirement of top level event, this paper provides a “top-down” fault tree analysis method to analyze the potential causes of top level event and build fault tree step by step, and explains the role of fault tree analysis Then the calculation of top level event probability is studied. By considering the influence factors of flight phase of top level event, the minimal cut set of fault tree and its probability, the failure rate of basic event and its risk time, the calculation model of top level event occurrence probability is provided. Meanwhile, the calculation model of top event probability is applied with an example to verify the feasibility of the calculation method of top event probability. The top event probability calculation model proposed in this paper can provide strong technical support for the risk control requirements of engine top-level fault events.