Abstract: In this paper, we use Bayesian decision to distinguish signal state from decoy state in one decoy-state quantum key distribution. The analysis results show that we can still only get the conclusion that the signal state and decoy state have the same i -photon transmittance, which verifies the rationality of the assumption that signal state and decoy state are indistinguishable, and also verifies the security of decoy-state method. In addition, by comparing the secure key rate of two cases including only signal state and signal state + one decoy state before and after photon number splitting attack, we find the former is successful and the latter is failed. This just proves the necessity of the existence of decoy state, verifies that the decoy-state method can indeed resist photon number splitting attack, and further verifies the security of decoy-state method.