The explosive growth in WWW is the main cause of Internet success. But because t he current Web servers architecture is short of QoS ensuring mechanism, it leads to significant revenue losses while server is overloaded. A QoS ensuring archit ecture for Web servers which provide multi-class service is presented in this pa per. It uses (SRD) service response delay as the core measurement criterion of t he architecture. With SRD, one class service is abstracted to a virtual server, and every component of a system are glued together. And a dynamic adaptive servi ce admission control algorithm (DASAC) is also presented. Experiments show that, although suffering severe overloading and burst, the user-perceived QoS can be ensured.