Abstract: The study of the starting mechanism is a prerequisite for mitigation of landslides. In this study, three-dimensional continuum model simulation method was applied by using friction model and Voellmy model to solve the runout process and to simulate the start-up process of debris-flow. The results show that, under the condition of both models and the erosion yield rate, whether the slope is a confined slope (channellized type) or an unconfined slope, with the increasing slope, the motion energy supplied by the topography further increases, the moving distance of the material also further increases, and the corresponding average speed, maximum speed (front speed), and the total kinetic energy will be further increased. The kinematic parameters experience the process from slow creep to rapid increase. Based on the start-up movement of landslide, the deposition process and variation of the kinematic parameters during runout under the condition of different slopes, the critical start-up angle of debris-flow is set at 25° -30°. The determination of the critical start-up angle can provide scientific basis for regional disaster prevention and reduction and scientific research.