In terms of the problem in calculating underground reservoir water storage space in the nearly horizontal coal seam at the coal mines in Western China,the calculation method,which uses the vertical displacement trajectory equa- tion of different boundary strata to calculate the difference in volume before and after caving,was put forward. The method could calculate the effective reservoir space of different rock zones and solve the problem of confirming the storage coefficient for calculating the underground reservoir water storage space. Based on the dividing results of frac- ture zoning of overlying strata,the main water storage space of underground reservoir was the gap,crack and separation space of caving strata and block hinged zone,and the calculation expression of effective water storage space in nearly horizontal coal seam was obtained. The three different structure forms,which might be formed in the boundary strata of caved zone,were analyzed based on structural mechanics and fracture mechanics theory. The vertical displacement traj- ectory equation of masonry beam structure,step beam structure and fully caving condition were studied. The theoretical calculation equations of effective water storage space of different rock structures were obtained. The fracture structure of overlying strata was analyzed using the UDEC numerical simulation calculation method based on the engineering practice of Daliuta coal mine. The path curves of vertical displacement boundary strata were obtained. The vertical dis- placement around the underground reservoir was smaller than the middle position,which means bigger storage space. The vertical displacement curves of each boundary rock strata were fitted and analyzed by using the method of theoreti- cal analysis and numerical simulation,and the effective water storage space of underground reservoir was obtained. The spatio-temporal evolution law of water storage space was analyzed based on the difference of vertical displacement curves of overlying strata in different calculation times. The feasibility and reliability of the calculation method for the effective storage space were verified by the field drainage test in Daliuta Coal Mine.