In view of the task capability evaluation in power supply system at an aircraft, a method based on statistical analysis and expert evaluation is proposed. Firstly, the power supply system at a certain type of aircraft is introduced, and its main failure modes and failure reasons are analyzed. Secondly, the Weibull distribution of power supply system faults is fitted through the statistical data from power supply system faults, and a method of calculating equipment fault probability during the task based on posterior knowledge is proposed to meet the requirements of equipment tasks. On this basis, according to the differences in the specific environment and intensity of task, the environmental factors in the power supply system are rated by using the method of expert experience scoring in comprehensive consideration of the vibration, load, natural environment and other factors. The results show that the proposed method can effectively predict the failure probability of the key components in the power supply system under the condition of fulfilling the mission, and comprehensively evaluate the mission environmental factors, supporting the decision of fulfilling the mission.