In this paper a new mechanical model indicating the mechanical behaviour of main floor in longwall mining is proposed. In the model the unfractured main floor is considered as an elasto-plastic plate, and the combination of fractured blocks as a voussoir beam. Using the plastic limit theory of plates, the limit load acting on main floor and the position of its largest deformation are gotten. The stability conditions for the key blocks of the voussoir beam are analysed by "S-R" sta-bility theory. The results of the theoretical analysis are important for the study on the water inrush from seam floor.