Abstract: A new calculating method for the mechanical behaviours of floors with underground water inrush inlongwall mining is proposed in this paper. The main floor under the influence of longwall mining can be considered asan elastic-plastic plate with various support conditions and the stability of the fractured plate can be analysed aVoussior Beam. Using the plastic limit theory of plates, the limit load of main floor and its largest deformation pointcan be obtained. By S-R stability theory,the stability conditions for the key blocks of the Voussior Beam of thefractured main floor also can be determined. These theoretical results are also important for the study on the water一inrush from floor in longwall mining.