Single-variable model is generally used for Apache Web server. It has poor scalability when it is used to handle multiple priorities delay guarantee. A Multi-Input Multi-Output (MIMO) system model for Web server was proposed with decentralized self-tuning controllers based on decentralized control theory and Self-Tuning Control (STC) theory. These controllers could dynamically adjust the number of worker threads, which handled different priority requests to ensure that the higher priority requests were processed faster and the average delay ratio of different priority was maintained the same as settings. The parameters of model and controllers were updated according to the result of online identification. The results of simulation tests indicate that proportional delay guarantee can be maintained on the Web server in the closed-loop system even if the number of concurrent client connections change abruptly under overload conditions.