The waste slag heap of mining could exert considerable effects on the hydrological process of a catchment. In this paper, a 2D numerical model of rainfall-runoff process based on a hydrodynamic approach was established to investigate the effects of the terrain change as a result of the slag heap on flood capacity in the Daxicha drainage basin, under the design storms with 5 different return periods. It is found that: the slag heap is able to attenuate the flood peak at the export sections. In the return period of five different types of rainfall, the storm with 10-year return period causes the most significant attenuation for flood peak, while for the storm with the return period less than 10 years, the terrain plays a more important role for the flood peak value and the arrival time, on the contrary, rainfall has become a major factor. The attenuation rate of the flood peak will increase as the section moves downstream. The research could help mitigate the flood risk in the flood plain downstream of the catchment with slag heaps.