Based on the theory of plates and shells, a mechanical model is established for high-located main key strata with two clamped edges and two simply supported edges. With Rayleigh-Ritz method, an analytical expression of flexural function and an expression of stress distribution are derived for high-located main key strata. Thus, the formulation of fracture span for high-located main key strata is solved and then the fracture laws under the same condition are further analyzed. According to microseism monitoring, the dependence of the fracture at high-located main key strata on microseism activities is investigated. It is shown that the point of maximum deflection before the key strata fractured is located near two simply supported edges. During the destabilized fracture process of high-located main key strata, fracture initially occurs along the dip clamped edge when the spanning coefficient prior to fracture is higher than 1, whereas fracture firstly happens along the strike clamped edge when the coefficient is lower than 1. Moreover, the relation curve of fracture span a and dip suspension length b fits in W-shape. Finally, it is found that microseism activities with high energy result from the fractures and movement of high-located main key strata. Furthermore, the predicted fractured step and format are in good agreement with the monitored microseismic activities.