To meet the need of how to depict the deformation behavior of covered rock strata in deep mining or inclined seam, the authors study the law of the bend of rock layers in coal mining district. Based on the elastic theory, the deformation character of the rock layer is studied, mechanical model to deformation of covered rock strata is founded, and an expression of flexibility equation and a theoretic formula of calculating the most flexibility spot are derived under the hypothesis that rock treated as rock plate. By calculation, the flexibility of worked-out area in mine lower part is bigger than upper part. The point of maximal flexibility lies in middle leaning down worked-out area in mine part, which reflects really characteristic of deformation of roof in worked-out area about deep mining of steep or inclined seam. The model reflects factually the deformation of rock boards under the action of the united stress in essence. The obtained results can, be used to provide science warranty to the calculation of covered rock strata in deep mining of steep or inclined seam.