Abstract: Based on rock structure of cantilever rock beam & hinged rock beam and the Winkler foundation model, this paper analyzes the causes of large area roo f cut and collapse of hydraulic supports, which occurs frequently at fully-mechanized top coal caving working faces in super thick coal seams. Then the paper proposes the measures to prevent and control such accidents. According to studies, hydraulic supports are crushed down and collapsed in large area by roof cutting down at the coal wall in the working face, which makes the immediate roof and the basic roof lose their horizontal balance. The reason of roof cutting tends to be low levels of settin g load and weak working resistance of the hydraulic supports, and accordingly they can hardly sustain the balance structure of immediate roof and basic roof. By increa sing setting load of the supports, their initiative support to the roof can be strengthened, and the sinking of roof can be lessened, thus being helpful to the balance of im mediate roof and basic roof. Through analysis with cantilever rock beam & hinged rock beam rock structure, large area of support collapse at Cuimu Coal Mine was due to the lack of working resistance of hydraulic supports at the working face.The paper proposes through theoretical calculation that the working resistance should have b een15000kN.