Abstract: There were bulk accumulation sediments of loose soil and rock in Beichuan area generated by Wenchuan earthquake on May 12, 2008, and continuous rainfall actions resulted in several times of severe landslides and debris flow disasters after a few months to few years of earthquake. Based on field investigation in geology, landform topography and rainfall data, a series of initiation of debris flow model tests under rainfall conditions in laboratory were performed respectively on Beichuan soil and silty fine sand sediments. Moreover, the effects of rainfall duration, rainfall density, slope angle and soil density on the initiation of debris flow were analyzed. Finally, the process and mechanism of debris flow initiation of Beichuan soil and silty fine sand under rainfall conditions were revealed. The results show that:(1) for Beichuan soil sediment, only the full infiltration stage occurs in the process of debris flow initiation and the critical rainfall density is needed much bigger in order to initiate debris flow.(2) for silty fine sand sediment, all the three stages of full infiltration, thin layer and deep surface seepage occur in the process of debris flow initiation, and the rainfall duration is needed much longer when the slope angle is bigger and the rainfall density is beyond a certain value.(3) the initiation of debris flow, whether for Beichuan soil or for silty fine sand, is the result of the combined action of gravity and seepage, but the function of gravity and seepage depends on the grain size distribution of soil. This research provides important reference data for early warning of debris flow disaster under rainfall conditions in Beichuan area.