Abstract: According to the seam conditions of Panxie Mining Area in Huainan, the paper establishes computational model of mining coal and rock. On the basis of analyzing coal and rock crack distribution characteristics, through the numerical simulation, the paper studies the change laws of B_4-1coal expansion deformation above goaf and the coal seam floor failure depth with different thickness of A3 protective seam; With the increasing of mining thickness, B_4-1coal seam expansion deformation shows exponential function tends, and behaves mutations when mining thickness is between 2 m and 3 m; in the cases of largest confined water pressure of 1.0 MPa, floor failure depth shows the logarithmic curve with the increase of mining thickness. Under the premise of that the protective layer A₃ can fully release protected seam B_4-1and coal seam A₃ can be mined successfully and safely above the floor confined water, the conclusion that the A₃ theory mining thickness will be among 2.21~ 4.0 m is drew.