Reliability evaluation during aero-engine operation is one of the key issues in the field of reliability evaluation of aero-engine, this paper proposes a method to support the evaluation of aeroengine operation reliability in the process of aircraft mission. The method is based on the characteristics of flight mission and aeroengine. Considering the current operating environment, the transient state of the aircraft, and the current working state of the engine, the quick access recorder (QAR) information is used as the analysis data to analyze the operational reliability. The proposed method combines Random Forest algorithm with stratified sampling method to fit, predict and calculate feature importance. Finally, a B737-800 flight mission from Beijing to Urumqi is taken as an example to verify the effectiveness and feasibility of the method. It solves the problem of data processing difficulty caused by large amount of data and high dimension in the process of aeroengine operation.