In order to study the mechanism of the influence of particle size on the start-up of slag debris flow,we took four debris flow gullies in Tongyu gold mining area of Shaanxi Province as the research objects. Through field investigation,laboratory test and numerical simulation,we revealed the mechanism of the influence of particle size on the start-up process of slag debris flow. The main conclusions reached are as follows:with the increase of fine particle content,the shear strength and permeability of slag decrease,and the effect of saturation state is more significant;the macro and microcosmic parameters of each grading slag are obtained by laboratory and numerical experiments;the grain grading of the slag can change the shear strength and the permeability of the slag body,and then influence the stress condition and the structure failure mode of the slag,and finally influence the start-up difficulty of the slag-type debris flow.