Abstract: Based on the gently inclined medium thick phosphate rock at the deep part of the east mining section of Jinning-Six Mine, Yunnan Phosphate Co. Ltd. from open pit mining to underground mining, floor friction model tests under the condition of different slope heights and boundary thicknesses were carried out with a floor friction model instrument. The results show that the deformation and failure response features of the slope can be divided into three stages:fracture and loose in a small range, unstable failure in a local range, and instability failure in the whole range of slope rock sliding to the goaf. The main deformation and failure modes of the slope are tension cracking, breakage, and sliding failure. When the slope height is fixed, with the boundary thickness gradually decreasing from 30 m to 20 m and 10 m, the stability of the slope is directly proportional to the boundary thickness, and the reasonable boundary thickness is 20 m to 30 m. When the boundary thickness is fixed, with the slope height increasing from 60 m to 108 m, the stability of the slope is inversely proportional to the slope height, and the slope of 108 m from open pit mining to underground mining is unstable.