According to pseudo-periodicity and ergodicity of chaotic systems, we put forward a fast algorithm to determine delay time in phase state reconstruction. The traditional method to calculate mutual information is time-consuming and complicated to realize, which greatly restricts its application. In our algorithm we gradually reduce the feasible computing interval and adjust the calculating step until satisfactory result is gotten. This method overcomes the tedious calculation of mutual information and meanwhile guarantees high precision. Finally the simulation results of Rössler and Lorenz systems verify the feasibility of our algorithm.