Abstract：The key stratum controls the deformation and failure modes of the mining rock mass. Based on mining condition which is covered by super-thick magmatic stratum，failure conditions and failure modes of magmatic stratum are studied by means of analytical solution—theory of thick plate rested on elastic foundation when mining is carried out in different stages. It indicates that overlying strata gradually break down and separation appears under the magmatic rock when the mining length increases. Tensile fractures could occur in the midpoint of the boundary in top side when mining at certain distance，then shear fracture could occur in the midpoint of the long sides in the central plan when the length of work face increases. After that，tensile fracture could occur in the midpoint of the up side and propagate to all round sides gradually if the length of work face keeps on increasing. At last，midpoint of the short boundary in the central plan fractures because of the shear stress along the thickness direction. The working face mining distance can decrease when fracture occurs and the mining width increases.
