Abstract: In order to determine the reasonable thickness of permanent sealing wall in Shanxi Datong Yungang Coal Mine 8617 fully mechanized caving return airway, we establish sealing wall mechanical model and determine the boundary conditions, and simulate the best ratio of fly ash filling material under different conditions by FLAC^3D software, and analyze the stress field, failure and displacement field related features of sealing wall. The result shows that optimum thickness of return air crossheading sealing wall at 8617 face in Yungang Mine is about 5.4 m.