Abstract: The utilization of underground goaf for water preservation and the underground reservoir technology for water recycling are the effective measures to achieve a water-preserved mining in western mining area of China. It is crucial for the safe and efficient implement of underground reservoir technology to research and ascertain the rational water storage capacity in the goaf. The key issues,such as the calculation of water storage capacity,the maximum and ration- al capacity of underground reservoir,have been investigated by theoretical analysis,stimulation experiment and field observation. It shows that the water storage capacity of underground reservoir is the summation of void content of bro- ken rock blocks in the overlying strata caving zone and the bed separation space of broken rock strata in the fissure zone within the water storage scope. The formula for calculating the void content of rock blocks in caving zone,which considers the coal seam dip angle,has been deduced based on the shape model resembling the parabolic body in the o- verlying strata caving zone. By utilizing the “O-shape circle” theory,the determination method of calculating the bed separation space under each key stratum and the broken rock strata between two key strata has been obtained. Thereby,according to the different locations of stored water level in overlying strata caving zone,the arithmetic expression for calculating water storage capacity in underground reservoir has been established,and the determination method of the maximum and rational capacity of underground reservoir has also been obtained. The capability of theoretical outcomes have been demonstrated by the practice of underground reservoir in Lijiahao coal mine.