In this paper a session-based admission time-ratio feedback control mechanism is presented for improving the Web QoS of commercial applications. The mechanism combines a novel session-based admission control (AC) policy with a PI controller. The admission control strategy manages the amount of requests that the server can accept, by dynamically changing the admitting time interval in the control period. During the admitting time interval all requests are admitted. The admission time ratio is naturally defined as the ratio of the admitting time interval in the control period. The novel admission control mechanism is agile and more easily implemented than adjusting the queue length. Moreover, PI controller designed using system identification and control theory makes this scheme robust and auto-tunable. The simulation results of Tomcat server provided in the paper demonstrate that it can improve the performance effectively and ease the resource control and QoS control.