Abstract: The study on the buckling failure mechanism of the floor of the steeply-inclined coal seam mining is of great significance for forecasting the buckling, sliding downward and protection of the floor. This paper discussed three failure modes of steeply inclined coal seam floor, including rotary sinking, directly sliding downward and buckling, and it focused on the mechanism of buckling. According to the occurrence characteristics of steeply inclined coal seam, the mechanical model of buckling failure of steeply inclined coal seam floor was obtained, and the displacement amplitude equation of bending uplift deformation of floor was derived. Through the analysis of instability properties of the floor after buckling based on the mutation and bifurcation theories, the bifurcation equation was obtained. The formulas of critical load, displacement amplitude and failure length at the moment of breakage were derived. The sensitivity of the critical load to the defect structure was analyzed, and the formula for calculating displacement amplitude was derived by its subdivision factor. The theoretical method for determining the critical load at bifurcation point and displacement amplitude through calculating the limiting fracture length of the floor, and its rationality was verified by an engineering example.