Abstract: In order to improve the efficiency and the accuracy of the electrical test for manned spacecraft, an automatic diagnostic system is proposed. In the expression of diagnostic knowledge of the system, multiple operators and functions are supported, to allow a high knowledge coverage. The inference engine is well designed, which fully takes into account the characteristics of manned spacecraft test to obtain high diagnostic accuracy and good real-time character. The automatic diagnostic system is now provided with functions such as parameter diagnosis, command diagnosis, event diagnosis and command monitoring, which would play a more important role in the electrical test for manned spacecraft.