This study collected and summaried the formation mechanism of debris flow. Firstly, based on the viewpoint of infinite slope, the critical movement model of debris flow developed by Takahashi(1977;1978) was discussed. The critical slope of debris flow is obtained by the known surface water level and seepege height. Secondly, the debris flow triggered by soil liquefaction was also discussed. The critical hydraulic gradient was obtained by the conditon of soil liquefaction. The condition of debris flow and soil liquefaction developed by Su and Chen (2003) was also discussed. Finally, the soil liquefaction due to seepage flow developed by Istomina(1957) was discussed. Based on the uniformity coefficients, the soil liquefaction can be classified into fluidization and scouring. Above discussions may be helpful for the understand of debris flow.