A method and a system for authenticating an entity based on a symmetric encryption algorithm are provided. The method includes the following steps: 1) an entity A sends an authentication request message to an entity B; 2) after receiving the authentication request message, the entity B sends an authentication response message to the entity A; 3) the entity A determines the validity of the entity B according to the received authentication response message. The implementation cost of the system can be reduced by using the authentication according to the invention.