Abstract: In order to reveal the overburden strata activity law under the fully-mechanized top coal caving mining in the horizontal sectional seam of the steep inclined ultra thick seam,a similar material simulation test was applied to study the overburden strata failure process,stress field variation and roof subsidence law of the steep incli ned thick seam after the mining in deferent horizontal sectional seam. The study showed that the fracturing of the overburden strata had a cracking occurrence,roof bed se paration,bed separation expansion,roof falling processes.Under the condition of the natural roof falling,a delay was existed in the overburden strata falling of the steep incli ned thick seam and the suspended roof breaking could cause a large area roof falling,roof strata bumping and other dynamic disaster.During the overburden strata falling a nd displacing,local stress of the roof would be distortion,a tensile stress would be appeared,the floor pressure stress would be increased in a linear and then until the roof f ailed and the stability lost.The roof separation quantity of the overburden strata would be steadily decreased from the bottom to the top.After the rock strata failed,a larger | ocation of the roof subsidence would be upward migrated along the inclined direction,until the surface ground subsided.