In order to ensure the reliability and safety of aviation equipment and improve the testing, maintenance and support capabilities of the system, higher requirements were proposed for system testability. The testability objective function model of the system was established by analyzing the testability problem of the system, considering the system function importance, failure rate, failure risk effect, testability and other factors. The application of functional importance analysis, functional failure risk and failure rate analysis, fault tree analysis and testability technology in the determination of testability targets for complex systems was studied. Through the analysis and calculation of testability target selection in auxiliary power unit (APU) control system, it is show that the proposed method can realize the analysis of functional importance and risk items in the power control system, and can guide the testability design of high risk items.