An entity authentication method by introducing an online third party includes the following steps: 1) an entity B sends a message 1 to an entity A; 2) the entity A sends a message 2 to a trusted third party TP after receiving the message 1; 3) the trusted third party TP checks the validity of the entity A after receiving the message 2; 4) the trusted third party TP returns a message 3 to the entity A after checking the validity of the entity A; 5) the entity A sends a message 4 to the entity B after receiving the message 3; 6) and the entity B performs validation after receiving the message 4. The online retrieval and authentication mechanism of the public key simplifies the operating condition of a protocol, and realizes validity identification of the network for the user through the authentication of the entity B to the entity A.