The invention discloses a bidirectional forwarding detection authentication safe switching method and device. The method comprises the steps that judging a current authentication state of BFD (bidirectional forwarding detection) messages by a first network device; sending two BFD messages to a second network device each time if the BFD messages are currently in the authentication intermediate state; and in the authentication intermediate state, when the first network device receives the BFD messages sent by the second network device, performing the following steps of judging whether detection of a first authentication mode is successful or not, refreshing a BFD overtime detection timer if the detection of the first authentication mode is successful, continuing to judge whether the detection of a second authentication mode is successful or not, refreshing the BFD overtime detection timer if the detection of the second authentication mode is successful, thereby enabling the second authentication mode to take effect, and logging out the authentication intermediate state. The problems of traffic interruption and route deletion resulting from modifying a BFD authentication mode in the prior art can be solved.