Diagnostic strategy design is an important task in the design for testability(DFT). In light of the fact that the practical tests are non-independency or costs of the texts are precedence-dependent, tests were considered to be distributed among multiple modes, and additional transition costs were considered if a test sequence involved mode changes. Then test costs and mode transition costs were combined to establish a heuristic evaluation function(HEF). Based on this HEF, a suboptimal diagnostic strategy searching algorithm was presented. Several cases were used to verify this method, and the experimental results show that this method efficiently solves the problem of diagnostic strategy generation with non-independent tests.