Abstract: In order to analyze the mechanical principle and hydraulic mechanism of channel debris flow under strong rainfall after earthquake, the authors take debris flow gully in Yindongzi of Longchi Town in Dujiangyan as a case study to establish the hydraulic models of underground water level change in loose accumulations. The characteristics and laws of ground water level change of loose accumulation in post-earthquake debris flow slope are analyzed according to the percolation theory in hydraulics, and the effects of hydrodynamic pressure and hydrostatic pressure on slope body are quantitatively studied. Through the combination of theoretical analysis, example verification and physical simulation test, a deep understanding of mechanical mechanism, critical condition and mode of start-up of loose source on slope was achieved. The results show that: ①Under the excitation of heavy rainfall, the hydraulic condition deteriorates continuously with the increasing diving level in the slope, which eventually leads to the stability failure of the loose accumulation body on the slope. ②The phreatic level generated by the accumulation body is a comprehensive function of slope area S, steady-state rainfall intensity I, slope geometric parameters (θ、W、H、Z) water conductivity coefficient T and other parameters. Tthe larger and I are, the higher the height of phreatic level H will be; Conversely, under the premise that the rainfall collecting area and steady-state rainfall intensity are determined, the larger T, W and θ are, the lower the phreatic level generated in the slope body will be. ③With the increasing hydrostatic pressure P in slope body, the sliding resistance of accumulation body base will be decreased; As the hydrodynamic pressure G_d inside the slope increase, the sliding force of the slope will increase. ④According to the existence form of residual sliding force, the starting mode of the loose accumulation body on the slope after earthquake can be divided into two types: the sliding mode of the whole slope starting and the traction mode of the disintegration in segments starting. The research results can provide theoretical and technical support for the advance warning of gully debris flow.In For the study of earthquake under the condition of strong rainfall channel debris flow mechanics and hydraulics mechanism, combining with the town of Dujiangyan order of the silver named debris flow gully is analyzed, the construction of slope are underground water level change under the premise of hydraulic model, with the aid of seepage theory of hydraulics, area after the earthquake were analyzed by slope loose accumulation is the characteristics and regularities of the variation of the groundwater level in the body, The effects of hydrodynamic pressure and hydrostatic pressure on slope body are quantitatively studied. Through the combination of theoretical analysis, example verification and physical simulation test,