Abstract: The reason of giant debris flow triggering was obtained by field investigation and mechanism of erosion in the watershed was explored with the aid of the finite difference software,FLAC^3D,while numerical simulation of stress field and displacement field as well as plastic zone of the system and analysis of stability in watershed under native stage was performed.Results show that the displacement of the watershed is mainly in the form of "compression deformation",its maximum displacement is mainly in loose gravel soil on the tops of slope-gully,as well as its parts is destroyed,and the vertical displacement is maximum.The changes of direction in the maximum principal stress of earth-rock boundary surface is more rapid than other region due to the existence of earth-rock boundary surface of gravel soil and limestone,the failure model in the watershed is both shear failure and tension model,the top of the layer in gravel soil both sides slope-gully is in the shear and tension plastic zones,its destructiveness is more serious.The modeling method and numerical simulation results can be applied to offer scientific basis for the debris flow predicting.