Abstract: The conditions of three soft coal seam in Huainan Mining Area are complex. During the process of mining overall height at one times in three soft thick coal seam (6 meters), using FLAC numerical simulation software systematically analyses the caving characteristic of the roof and floor with the increase of face advacing distance. As the working face just begins to advance, the range of roof's vertical displacement is bigger than floor's, but the maximum is less. As mining goes on, the maximum of roof's vertical displacement is gradually close and greater than floor's, when the working face advances to 30 m, the maximum is roughly equal. Workers should strengthen the monitoring and controlling for the coal wall in front of the working face to prevent the large area wall caving. Earlier stage, prevention and control of the floor still should be noticed.