The hydrogeological conditions of Weibei coalfield in China are complex; the main mining No. 5 coal seam is seriously threatened by Ordovician limestone karst water disaster at the bottom of coal measures. Chenghe No. 2 Mine is a typical example. With the increase of mining depth, confined water pressure increases year by year. To find out the law of floor deformation and failure caused by No. 5 coal seam mining in Chenghe No. 2 Coal Mine, this paper takes 24,508 working face of Chenghe No. 2 Coal Mine as engineering geological background, and carries out research by numerical simulation and field test methods. The stress field, displacement field distribution law and plastic zone evolution characteristics of No. 5 coal seam roof and floor varying with the advancing degree of mining face are obtained by simulation calculation. The progressive failure process of the whole floor stratum is reproduced dynamically, and the development height of the water conducting fracture zone of overburden is given. The maximum failure depth of the floor occurs at the mining distance of about 1.5 times the mining width, at which time “the saddle shape” supporting pressure arch reaches its maximum. At the same time, the multi-point separated layer displacement meter is used to carry out field measurements. The results show that the maximum failure depth of floor occurs in the goaf, and the maximum range of the relative displacement of the floor is 8.0–8.5 m, which is close to − 6.1 m from the coal mining surface, consistent with the results of the numerical simulation. Comprehensive analysis shows that the maximum failure depth of coal seam floor in Chenghe No. 2 Coal Mine is 8.3 m. The conclusion provides a favorable basis for the rational formulation of water disaster control countermeasures. It provides reference and experience for mining under safe water pressure of aquifuge for prevention and cure water of similar working face in Chenghe mining area in the future.