Abstract: For the problems of big step and large fall ground fissure appearing on surface after top-coal caving mining at 20 m extremely thick coal seam under thick layer overburden, by taking means of integrated geophysics in ground fissure, height measurement of fissured zone, we analyze the formation mechanism of strata displacement law, and find that the deep of ground fissure develops into close shear cracks without hydraulic conductivity; bedrock above fissured zone is overall pressured off; the bending zone usually replaced by whole fault zone, which broken in middle and both ends of the rock beam. Finally, a series of measures proposed for geologic hazard prevent.