To realize the efficient utilization and zero discharge of mine water, constructing underground coal mine reservoirs by adopting the goaf space formed after the mining of contiguous coal seam groups is one of the effective measures. Nevertheless, determining the spatial distribution law of goaf porosity and calculating the storage capacity of underground reservoirs accordingly is the key to evaluating the water storage capacity of underground coal mine reservoirs and ensuring their safe and stable operation. Accordingly, the underground reservoir in the first mining panel of Lingxin Coal Mine is taken as the engineering research object, and the FLAC3D numerical simulation method is adopted to analyze the morphological characteristics and development height of the water flowing fractured zone after the mining of contiguous coal seam groups. Meanwhile, the water storage space of the underground reservoir is determined according to the scope of the water flowing fractured zone in the goaf. On the basis of the correlation between porosity and bulking coefficient, the spatial distribution law of porosity in the water flowing fractured zone is clarified, and the theoretical storage capacity of the underground coal mine reservoir is further calculated. The study results indicate:(1) Due to the contiguous multi-seam mining in the first mining panel, the failure scope of overlying strata presents superposition and expansion, and the maximum development heights of the caving zone and water flowing fractured zone are 37 m and 115 m, respectively.(2) The porosity of the caving zone is 0.25, which shows good water storage performance, and the porosity of the fracture zone ranges from 0.09 to 0.25 from top to bottom, with the water storage performance of the fracture zone gradually decreasing from bottom to top.(3) As the hydraulic head in the reservoir rises, the capacity of the underground reservoir initially increases slowly and then increases rapidly in a linear fashion. Eventually, the capacity gradually stabilizes due to the decrease in porosity at the upper part of the fracture zone, with a maximum storage capacity of 7.9×106 m3. In summary, the research findings provide a theoretical foundation for the construction of underground reservoirs and the safe and efficient storage of high-salinity water in western mining areas.