Abstract: In view of the requirement of design for testability, based on the IEEE Std 1149.4, a optimize method of the replacement and interposition of boundary scan components is proposed by using the relevance model to analyze and make modeling for a mixed-signal system, and building the dependency matrix between the parts of the circuits and the boundary scan test. A universal design of BIT is presented by making the corresponding diagnostic strategy. The verifying experiment of system indicates that the optimize method can make test rapidly and promote the testability of electronic system efficaciously.