This paper used Particle Flow Code (PFC3D) to simulate the model experiment of the process of the initiation of debris flow. Based on the previous simulation studies and considering the unsaturated features of sand soil, it simulated water with small granules in the numerical simulation of the initiation phase of sandy debris flow, and set a bond model to simulate the matrix suction between soil particles. Compared with the results of the numerical simulation with laboratory model tests, the result of the study shows that the start of debris flow forms with rear soil pushing the front soil rapid decline. Under the condition of unsaturated, the matrix suction between soil particles provides soil strength. When the penetration and gravity are beyond the bond force between particles, soil turns from unsaturated to saturated and debris flow will start. Compared with the results in the simulated method which does not consider the condition of unsaturated, the method of considering the unsaturated is suitable to simulate the initiation process and failure mode of flow-slip mudslides.