Abstract: The long-wall method was used to mine a kind of gently inclined and extremely thin iron ore deposit. A similar material model was built for the iron ore deposit, the deformation, stress and failure of cap rock were monitored with an electronic total station, strain gauges and digital cameras. The collapse law, the variation of stress and displacement, and the deformation mechanism of stiff cap rock were studied during mining the gently inclined and extremely thin iron ore mine by long-wall method. Experimental results show that the collapse of cap rock is sharply intermittent and jumping, the fracture pace of the collapse roof is the same as previous steps, and the orientation of fracture is almost parallel. In the vertical orientation, the fracture pace relates with the location of rock. The fracture period and the movement law of cap rock were also obtained.