Abstract: For thick coal seam at mechanized caving mining conditions of coal project example, Analysis the perturbation failure law of overlying strata and explore the basic conditions for the formation of roof separation space. Discriminate cavity type separation location of the study area, obtained a total of 5 roof rock can develop the cavity type separation space. Draw the fitting curve of the relationship between mining height and the ratio of the height of the fractured zone to the mining height by the empirical formula for thick coal seam, then draw the maximum development highly evaluates 221.8m for water flowing fractured zone of overlying strata. Simulate the coal mining process by numerical simulation of FLAC3D, draw vertical displacement of roof rock changes, verify the correctness of the theory.