Using the self-developed visualization test apparatus, centrifuge model tests at 20g were carried out to research the macro and microscopic formation mechanism of coarse sand debris flows. Theformationmodeand soil-water interaction mechanism of the debris flows were analyzed from both macroscopic and microscopic pointsof view respectively usinghigh digital imaging equipment and micro-structure analysis software Geodip. The test results indicate that the forming process of debris flow mainly consists of three stages, namely the infiltration and softening stage, theoverall slide stage,and debris flow stage. Theessenceof simulated coarse sand slope forming debris flow is that local fluidization cause slope to wholly slide. The movement of small particles forms a transient stagnantlayer with increasing saturation, causing soil shear strength lost and local fluidization. When the driving force of the saturated soil exceeds the resisting force, debris flowhappensonthecoarse sand slope immediately.