Military aircrafts show the phenomenon of insufficient diagnostic ability in the outfield, which seriously affects the efficiency of aircraft redeployment. In this paper, based on the practical application requirements of fault diagnosis of military aircraft, the testability design problems of airborne system in the process of design and test are analyzed, and the key links in the process of testability design, such as diagnostic capability design and testability test, are analyzed.Therefore, a complete design system for airborne system test is proposed. Aiming at the whole development and use process of military aircraft, the design objectives are defined, and the design, verification and evaluation methods are provided. The original isolated design work is integrated into a complete design system. At the same time, the improvement suggestions on working methods, professional positioning and system support can effectively promote the integration of testability design into the process of airborne system design, improve the testability level of airborne system, and improve the comprehensive diagnosis ability of aircraft.