Abstract: Based on software FLAC^3D, we simulates the distribution law of elastic strain energy in thickness variation of coal seam when the thickness of coal seam varies from 5 m to 1 m in 10 meters, and analyze its influence factors. The result shows that the elastic strain energy density will increase in coal seam thinning place and decrease in coal seam thickening place. This phenomenon will be more obvious when the elastic modulus of coal seam is lower, the coal seam roof and floor is harder and the thickness changing of coal is more severe.