In order to study the initiation mechanism of non-equigranular particles on loose soil slope under the runoff-seepage coupling effect， a stability analysis model of particles is constructed based on sliding and rolling mechanical analyses of single particle， and the Navier–Stokes Equation and Brinkman extended Darcy Equation are used to describe slope runoff and seepage respectively. Combined with the continuity equation and stress jump conditions， the concrete expression of the fluid-solid interface velocity is calculated， and stability safety factors of single particle under the runoff-seepage linkage effect is obtained. The results show that stability safety factors of particles are mainly affected by runoff depth on the slope surface， thickness of loose layers， slope gradient， stress jump coefficients， soil internal friction angles， equivalent diameters and volumetric weight of particles， porosity and permeability of loose soil slopes； slope gradients， soil internal friction angles and volumetric weight of particles are sensitive parameters of anti-slide safety factors； slope gradients， volumetric weight of particles， runoff depth on the slope surface and permeability of loose soil slope are sensitive parameters of anti-roll safety factors.