Debris flow has caused serious human casualties and economic losses in the main earthquake-hit areas affected by the Wenchuan earthquake. As one of the important and effective means of non-engineering disaster mitigation, debris flow hazard assessment is a key issue for the sustainable economic and social development of earthquake-hit areas. This article illustrates a new method to quantify the debris flow hazard by combining debris flow simulated results with different land utilization within the influence area. The Guo Juanyan gully in Dujiangyan city, Sichuan province, China, located in the meizoseismal area of the Wenchuan earthquake, was selected as the study area. The rainfall characteristics, including the rainfall pattern and the 10-min, 1-h, and 24-h critical rainfalls, were fully explored first. Then, the numerical simulation method was applied as a modeling tool to simulate debris flow influence area and final buried depth under rainfall with different return periods. The simulated results under a 100-year return period rainfall were validated based on field measurements. Finally, the debris flow hazard maps under different return periods were overlapped by combining the simulated results and the types of land utilization. The proposed method can enhance the accuracy of debris flow hazard assessment and can be widely used for debris flow mitigation, which has important application value.