BIT is a new technology about the system test, fault diagnosing and separating. It is more important in the design for the reliability and main tainability of system. The key technique, such as the fault tree and equation, the test location, the test signal picking and generating, the fault distinguish rule, the design of preventing error alarm and BIT software. The result showed that the key technology is useful, and the reliability, maintainability and ability of management to fault can be increased by means of BIT. The debugging of system becomes convenient. The method can be used in the system of intelligent-in-strument and industrial control computer.