The adaptive optics (AO) system involves many fields like optics, electronic control and so on, so it is necessary to improve the reliability of the electronic control system. The fault tree analysis (FTA) method introduced, and the key modules, such as the wavefront detection unit, wavefront processing unit, are analyzed.A fault tree model of the system is established. The minimum cut sets of system are got through qualitative analysis. Similarly, the probability of failure of the system and the order of the importance of bottom incident are got through quantitative analysis. According to the result, the stability of system can be improved and the design of system can be optimized effectively.