A planar 2D mathematical model of the shallow water equations for solving dam-break flood flow problems is presented based on the reverse format. A good method of dealing with wet and dry boundary in calculation is given in the article, so the movable boundary simulation problem is solved successfully. And the dynamic display of the flood routing comes true by treating module. Taking the measured data of partial dam-break models given by Fraccrollo et al as the examination of mathematical model, the results show that the calculated water elevations agree well with the experimental results except near the dam site at the beginning of dam-break. In the end, the model is applied in the calculation of natural dry-bed river flood routing.