Abstract: The covering layer of transition from open-pit to underground plays an important role in preventing the air leak,seepage of water and pressure bump.The movement mechanism study on layer will be beneficial to control the structure of layer and to ensure the smooth of underground mining.It also has great significance for the transformation safety of mines.The numerical simulation was developed by using PFC2D combined with the structure parameters of underground stope and the composition parameters of the layer in a certain mine to explore the movement speed change law of internal granular particles.It is shown that the average movement velocity of small particles is 1.6 to 2.0 times of big particles,and the velocity of particles above route is 2.5 to 3.0 times of other region.At the same time,the velocity variation trend of particles above route is faster relatively in the late phase of ore-drawing and the other region is relatively flat.

Key words: Covering layer, Granules, Pillarless sublevel caving, PFC