On May 12, 2008, the Wenchuan earthquake in Sichuan Province, China caused surface damage, frequent landslides and debris flow, which further exacerbated the flash flood hazards. Two giant debris flows in Longxi River basin (located in Wenchuan county) occurred due to heavy rainfall after the earthquake: one was on August 13,2010, and the other on August 18,2010. Reduction and risk assessment of flash floods are the key issues in post-disaster reconstruction. Due to lack of sufficient research data, hydrological prediction models are employed as important and cost-efficient mitigation tools. Wenchuan earthquake area was taken as study site in order to illustrate the influence of geographical environmental changes on micro-mechanism of mountain floods. In this paper, hydrological observation and simulations using remote sensing data and the WMS HEC-1 model are carried out in the typical flood-hitting area (Jianping Gully basin, Dujiangyan City, Sichuan Province, China). The hydrological response of rainfall runoff is discussed. Precipitation over the study area affects the runoff and floods, however precipitation has positive relation with the peak flood discharge. The simulation shows that the peak flood and runoff volume on August 13, 2010 is close to a 10-year maximum. The flash flood was the main cause of the large debris flow on that event. However, peak runoff remains the same regardless of the intensity of rainfall, which may be due to the shape of the river basin and the initial conditions of the model setting. This paper can get better understanding of the forming mechanism, evolution process, distribution and assessment of torrential flood induced by a devastating earthquake.