The BIT technique is an important approach to improve the testability and diagnostic capability of the system. But the high false alarm rate(FAR) is one of the important factors that prevent BIT from wider application. The states of a system are divided into 3 states that include OK state, intermittent state and faulty state. The Markov model is set up, the relation between 3-state model and 2-state model is compared. Theoretical analysis shows that BIT of 3-state model is better than that of 2-state model in reducing FAR as well as increasing false diagnosis rate(FDR).