An entity bidirectional authentication method and system, the method involves: the first entity sends the first message; the second entity sends the second message to the credible third party after receiving the said first message; the said credible third party returns the third message after receiving the second message; the said second entity sends the fourth message after receiving the third message and verifying it; the said first entity receives the said fourth message and verifies it, completes the authentication. Compared with the conventional authentication mechanism, the invention defines an on-line retrieval and authentication mechanism of a public key, realizes the centralized management for it, simplifies the operating condition of the protocol, and facilitates the application and implement.