In this paper, the key stratum theory is introduced in view of the small surface deformation value of the mining of thick coal mines in Gucheng Coal Mine. The analysis of geological drilling data in Gucheng Goal Mine determines three key strata groups and one main key stratum for controlling the surface deformation of the stratum above the No.3 coal according to the stiffness and strength condition of the stratum. The breaking distance of the main key stratum in this Mine is determined to be 302m-373.7m in terms of the elastic foundation beam theory and the surface measured basin data. In addition, a large-scale separation model of the main key stratum of the five strip mining face of this Mine is established, and the quantitative calculation formula of the limit spanning of rock beam is proposed and applied to. Finally, the probability integral method is used to predict the surface deformation problem of multiple working faces, and the parti-tion prediction is performed according to the key stratum breaking distance to obtain prediction values of surface deformation consistent with the fracture of key stratum in the study area.