This paper analyzed the steps of the early warning system to obtain the influencing factors of the early warning time. By comparing with the early warning delay decomposition of the Swiss Seismic Network we obtain that the instrument delay time of the earthquake early warning system is 7.6s in China. In this paper we present an algorithm of seismic wave arrival time,which is more accurate than the traditional algorithm with the epicenter assumed to be in the center of the station. Under this algorithm,the time required for receiving seismic waves by the two-station method and the four-station method is calculated to obtain the delay time of earthquake early warning in China. We discuss the influence of the two-station method and the four-station method on the size of the early warning blind area under different focal depths and station densities. We find that when the interval between the stations is less than 20km,the difference between the two methods of early warning blind area is not significant. Through theoretical calculation,when the station density reaches a certain density,the instrument delay time of the early warning system is shortened,which is more effective than the station encryption to reduce the warning blind area.