Abstract: The underground pressure behavior is stronger at fully-mechanized coal face in extra thick coal seam and knowledge about relationship between mining pressure and mining thickness is insufficient.This paper studied the fully-mechanized coal face in No.3106 extra thick coal seam of Luizgou Coal Mine.First of all,working resistance of suppurt at two different high-intensive exploitation extra thick coal seams,which were 17 m and 30 m respectively,were tested and it was found that two underground pressure behaviors shared similarities.Then,an indoor physical similar simulation experiment was made by building a mechanical model of overlying rock in large space under two conditions.By analyzing its action mechanism on underground pressure behavior,it is indicated that in the mining thickness of 30 m,the overlaying strata features "the lower coal seam supporting the cantilever beam-the upper coal seam supporting the free beam-the main roof jointing the rock beam".The special construction makesthe immediate roof and the main roof deformed by rotation,and their acting force to brackets decrease,which explains why the underground pressure do not get stronger when the coal seam get thinker.