As the pre-launch testing process is very complex and the number of critical parameters and fault modes in monitor are quite large, which brings command staffs a lot of burdens and pressures coming from fault diagnosis, a general test platform with fault diagnosis is used in launch vehicles’ pre-launch testing process. By a requirement analysis of diagnosis function for the platform, a fault diagnosis system for the platform is proposed in this paper, including system architecture, BIT, process monitoring, fault location, and information application. With this intelligent function, a quick automatic testing system can be used in pre-launch testing process, and efficient auxiliary information can be provided for maintenance decision making.