The water curtain hole design is the core part of water curtain system, which is crucial to the operation of the underground water-sealed cavern. Based on the first water-sealed underground cavern in China, the impacts of spacing of water curtain hole and fracture width on the water sealing is studied using the underground water seepage theory. The results show that spacing of horizontal water curtain hole and the fracture widths have important influences on water sealing in steep-inclined structural planes. In the calculation condition, the water sealing reaches the best effect when the water curtain hole spacing is 10 m. Water curtain hole spacing of 30 m is unable to meet the requirements of water sealing. Under the same water curtain hole spacing, the wider the gap width is, the better the effect of water sealing is, the best spacing of water curtain hole is 10 m for the project investigated; the poorer the effect of water sealing is in the crown area of main cavern. The research results provide a theoretical basis for water sealing evaluation of water-sealed oil storage cavern