An entity bidirectional 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; 3) the trusted third party TP verifies the validities of the entity A and the entity B; 4) after verifying the validities of the entity A and the entity B, the trusted third party TP returns a message 3 to the entity A; 5) the entity A sends a message 4 to the entity B; 6) after receiving the message 4, the entity B performs the verification to complete the authentication for the entity A; 7) the entity B sends a message 5 to the entity A; 8) after receiving the message 5, the entity A performs the verification to complete the authentication for the entity B.