This paper studied on the activation time of sprinkler and the critical heat release rate to activate the sprinkler when a fire happened. By calculation, this paper gave the curve on the activation time and the factors which would impact the activation time. The following conclusions were done： the activation time was almost linear with the installation height and the environment temperature; the fire development mode would affect the increasing rate of the activation time; the exponential relationship was found between the activation time and the response time index (RTI). As the height increased, the critical heat release rate would increase. In order to control fire effectively, the fast response sprinkler must be used when the sprinkler was installed in a high building ceiling.