Landslide surge is a grave disaster in the operation of reservoir. The prediction of landslide surge requires an analysis of the sliding speed of the landslide. With a high-level loose landslide body in a reservoir as an example, the sliding speed of the landslide body was calculated using traditional empirical formula and discrete element method (DEM) respectively. Traditional method has some limitations in calculating the sliding speed of loose landslides, while DEM is superior in analyzing the process of landslide surge of high-level loose landslide as the DEM-simulated deformation process is basically consistent with qualitative instability process. According to DEM-calculated deformation and failure process of landslide at different instances and the velocity of DEM spherical particles,the speed of unstable materials was not large in the incipience of sliding under the action of gravity, but increased continuously during the movement on bedrock slope, and finally reached about 40 m/s upon entering water. The present method has advantages in analyzing the disaster process of high-level loose landslides and provides a good reference for similar engineering problems.