A method and system for entity public key acquiring, certificate validation and authentication by introducing an online credible third party is disclosed. The method includes the following steps: 1) an entity B transmits a message 1 to an entity A; 2) the entity A transmits a message 2 to a credible third party TP after receiving the message 1; 3) the credible third party TP determines the response RepTA after receiving the message 2; 4) the credible third party TP returns a message 3 to the entity A; 5) the entity A returns a message 4 to the entity B after receiving the message 3; 6) the entity B receives the message 4; 7) the entity B transmits a message 5 to the entity A; 8) the entity A receives the message 5. The present invention can achieve public key acquisition, certificate validation and authentication of the entity by integrating them in one protocol, thereby facilitate the execution efficiency and the effect of the protocol and facilitate the combination with various public key acquisition and public key certificate state enquiry protocols. The present invention suits with a “user-access point-server” access network structure to meet the authentication requirement of the access network.