Abstract: It is of great significance to study the floor failure depth in the process of coal seam mining for the prevention of water disaster to mine floor. Numerical simulation can simulate and predict the failure depth before coal seam mining, which can provide reference for mining work, because it avoids the situation that it cannot be measured due to complex site conditions. Taking 32021 working face of Liangbei Coal Mine as an example, the theoretical calculation is made based on the empirical formula of statistical law, and the obtained floor failure depth is 27.05 m, 25.63 m and 23.56 m. Considering the mining depth and the coal seam dip angle, 27.05 m is closer to the actual situation. FLAC^3D is used for numerical simulation to simulate and analyze the stress variation law and the failure depth of the floor during the advancing process of the working face. In the process of numerical simulation, the change of vertical stress of the coal seam floor shows that with the continuous advance of the working surface, the stress on the roof and floor increases, and the phenomenon of stress concentration occurs at both ends of the goaf, the influence range increases, and the failure depth of the floor also increases. The plastic failure results of coal seam floor show that the floor failure depth increases with the advance of working face. When the working face advances to 100 m, the floor failure depth reaches the maximum, and then the failure depth remains unchanged and the failure range will increase. When the working face is advanced by 100 m, the plastic failure results of different depths under the coal seam floor show that the failure range decreases with the increase of depth, and the maximum failure depth of the coal seam floor can be judged to be about 25.9 m.