Abstract: In order to explore the deformation and failure depth of the floor under the condition of fully mechanized top coal caving of extra-thick coal seam, this paper takes 1305 working face of Dongtan Coal Mine of Yankuang Energy Group Co., Ltd. as the background. The deformation and failure depth of the floor in the mining coal seam of the working face is comprehensively analyzed by using field measurement, numerical simulation and theoretical calculation. The field measurement results using the strain induction method and borehole imaging technology show the following results. The floor is affected by mining ground pressure, and there are obvious characteristics in horizontal and vertical directions. In the horizontal direction, the position near the advanced support measuring point of 50 m and at the depth of 10 m in the shallow part of the floor starts to be affected by the mining ground pressure. After the working face is pushed over a certain distance, the deformation and failure of the floor are severe. The variation range of crossover distance and lag distance in the horizontal direction of different depths of the floor is 96-115 m and 48-52 m respectively. The deformation and failure depth of the floor in fully mechanized top coal caving of the working face is 16-20 m. The floor rock below the vertical depth of 20 m is mainly elastic deformation. The distribution characteristics of the plastic zone in different depths of the floor by numerical simulation show that the farther the distance from the working face floor is, the smaller the influence of mining pressure is, and the smaller the range of the plastic zone is. The 20 m under the floor is basically not damaged. The result of the theoretical calculation confirms that the deformation and failure depth of the floor is 19.2 m. Based on the results of field measurement, numerical simulation and theoretical calculation, the deformation and failure depth of the floor in 1305 working face is less than 20 m. The research results can provide the quantitative basis for the prevention and control of floor water disasters in fully mechanized top coal caving of extra-thick coal seams.