Abstract: In order to study the mechanical mechanism of the instability of the reverse fault in the working face under the condition of thick and hard roof, the spatial relationship model between the overburden movement area and the fault plane was established through the relationship among the size of the coal pillar at the fault boundary, the fault dip angle and the angle of rock strata movement. The equation for the movement line of overlying strata was established based on the movement angle of each rock layer in the mining area. The distance function formula between the movement line of different layers of overlying strata and the fault plane was obtained by combining the fault dip angle, and the boundary transition area between the fault plane and the rock layer movement line was accurately delineated. A limit equilibrium equation for boundary rock blocks was established based on the tensile strength of rock layers, and a mechanical instability criterion for boundary rock blocks was given when the fault dip angle was less than the rock layer movement angle. Based on on-site cases, the activation rock zone of reverse faults with a dip angle less than the rock movement angle was determined using dual criteria, and the activation law of fault planes and the spatial distribution of separation layers were determined. The deformation and failure mechanism of sur-rounding rock under the boundary condition of reverse fault is theoretically analyzed. The overall migration law of rock strata in the fault area is simulated by experiments. The mechanical and displacement variation modes of surrounding rock in the reverse fault area under the condition of thick and hard roof are comprehensively calculated and explored. The results show that the reverse fault is activated before the breaking of the thick and hard roof. When the fault is less than the dip angle of the rock stratum, the separation space of the fault surface is mainly distributed in the bending subsidence zone of the thick and hard roof area. The vertical fracture space of the rock stratum which is separated from the fault plane is significantly reduced. the upward transfer efficiency of the rock layer that generates the separation space with the fault surface to the goaf space is significantly improved. The stability of fault surrounding rock can be maintained by optimizing the size of fault protection coal pillar. The double criterion model gives the size value basis of the fault boundary protection coal pillar from the theoretical level, and the optimal solution of the field protection coal pillar width is provided.