Based on the high mining height of large cutting height workface in shallow thick coal seam and the few falling waste rock in goaf, the key roof can not be supported effectively, the facture mechanical model of key roof was established. The theoretical calculation formula of key stratum fracture step and working resistance of support were obtained by using fracture mechanics. The results show that the fracture step of key roof relate to not only the mechanical character of key roof and the load of overlaying rock seam, but also the working resistance of support and horizontal pressure in key roof. The reasonable working resistance of support and the step of roof fracture were analyzed in 1-2coalmine 51104 face of a mine in Dongsheng area. The theoretical results are well agreeable with the field measured results.