Abstract: Based on the explicit difference scheme of one-dimensional Saint-Venant water equation, a new explicit difference scheme is deduced to establish the evolutionary routing model of flood considering the bottom slope and the resistance to water flow. The water depth, current velocity and other parameters of flood at different time and distance during propagation process can be calculated with this model. Based on the data of water depth, current velocity and impact time, the computational formula about the impact loading of flood model is deduced by combining the one-dimensional unsteady flow energy equation and the analytic solution of flood routing. The impact loading of flood at different time and distance can be calculated simply with this formula. The impact loading changes linearly with the depth of water and relates to the upstream static level and distance, the height of the impact face and the impact time. Finally, the numerical analysis is carried out through calculating the impact loading of three cases and discussing its variation rule.