Abstract: In the uniform iterative algorithm for hydrodynamic flood routing model, the iteration step and the actual required time of each grid are inconsistent, and directly affects the precision of water level and discharge estimation for each grid cell. To solve this problem, a double-layer asynchronous iterative algorithm is proposed. The outer iterative process controls the flood routing time, while the inner iterative process adaptively selects the step size and the times for iterations by analyzing the flood velocity characteristics of different grid cells. FloodArea, the uniform iterative algorithm and the double-layer asynchronous iterative algorithm are applied to simulate historical heavy rain flood processes in the Wanan basin in Fujian province. Experimental results show that the simulated results of the double-layer asynchronous iterative algorithm are most consistent with actual disaster data. The average error in the proposed algorithm is less than FloodArea, 0.361 m and less than the uniform iterative algorithm, 0.654 m. The proposed method effectively improves the integral accuracy of flood routing simulation.