Abstract: The deformation and fracture characteristics of hard and thick key strata affect the movement of the overlying strata. In view of the poor adaptability when the traditional “beam” or “thin plate” theory is used to analyze the stress and fracture of thick and hard key strata,based on the theory of medium and thick plate,the displacement and stress distribution of the key strata of the isolated island face with thick and hard key strata are studied,the influence of the thickness of key strata on the distribution of shear stress is discussed,the stress and fracture characteristics of key strata are revealed,and the criterion of failure mode of key strata is proposed. The results show that under certain overburden conditions,the smaller the thickness of the key strata,the more prone to tensile rupture. With the increase of the thickness of the key strata,the failure mode gradually changes into tensile shear mixture failure and shear failure. When the thin key strata are damaged,it is often broken by stretching. After the key strata is broken,it rotates along the fulcrum to form an articulated structure,which will not cause too much load on the working face. The hard and thick key strata is often cut off and broken,causing impact on the working face. The thin key strata are more prone to tensile rupture,and the broken key strata rotates along the fulcrum to form a splice structure,which will not cause too much load on the working face. The hard and thick key strata tend to shear failure,causing impact on the working face. The failure location of the hard and thick key strata prone to shear fracture is different from that by the traditional“thin plate” theory,and the distribution of shear stress in the hard and thick key strata is more concentrated. Th shear stress distributed in the hard and thick key strata increases with the increase of the thickness of the key strata.Before and after the weighting,the strain energy peak inside the key strata are transferred from the middle to both ends of the working face. The distribution of shear stress in the hard and thick key strata is more concentrated. The focus of ground control is to focus on the area where shear stress is concentrated,so as to realize the prevention and control of coal and rock disasters in working face in different regions and grades.