The frequency of extreme rainfall corresponding with debris flows and large-scale landslides is increasing in the Taiwan. The traveling distance of debris flows get longer with increased sediment supply, and the hazard area is also larger than regular disaster situation. The debris flows can be considering as a mixture with solid phase and liquid phase. With a larger sediment supply in the dynamic process, the volume of fine sediment separating from solid phase is also larger, and so the state of liquid phase is change after fine sediment mix with water. This phenomenon contributes a significant effect on the mobility of debris flows and large-scale landslides. The composition of the mixture plays an important role in the analysis of the hazard area. In the present study, a two-dimensional numerical model is proposed to simulate the dynamic process of debris flows and landslides under different compositions of the mixture. The proposed was applied to simulate a case of extreme rainfall event, and the topographic data after disaster was used for verification. When the composition of the mixture included with fine sediment, the simulation of affected area had a good agreement with the real situation. The present study pointed out that the simulation of debris and landslide under extreme rainfall condition should consider the effect of fine sediment in the simulation condition.