An adaptive session-granularity admission control (SGAC) method, which combines the session and request granularities, is proposed for flash crowd control. In SGAC, the average response delay is used to measure, detect, and control the flash crowd. Once a session is allowed to access the server, it will be served until it ends. Besides preventing a server from overloading, SGAC can protect the sessions’ integrity. By regulating the session served number adaptively, SGAC can improve the server’s utilization. The performance of SGAC with real HTTP log are evaluated, and the result show that SGAC can effectively prevent servers from overloading, protect sessions’ integrity, improve server’s utilization, reduce the request arrival rate, reduce the access router’s computing overhead, and protect valuable transaction sessions.