A method and system for ensuring QoS in a communication network and a SLA server. The system for ensuring QoS includes a service-specific server and a SLA server. The method for ensuring QoS includes: querying the SLA information according to a user identity, where the SLA information includes different service levels signed by the same user identity; obtaining different service levels signed by the same user identity; and providing the user with the service at a corresponding service level. Through the technical solution of the present invention, a user can subscribe to more than one service level and the SP can ensure the QoS at different service levels for the user.