Abstract: The water sealing criterion is a base for the design and operational management of underground rock caverns for oil storage. To obtain a simple and practical criterion to evaluate the water sealing effect of unlined underground rock caverns for oil storage, a gas escaping model is established for describing the migration of fracture water owing to high gas pressure based on the researches on resistance force acting on the fracture water. According to the critical conditions of fracture water movement, a formula for calculating the critical water sealing height satisfying the gas sealing condition is deduced. Furthermore, the water sealing effect of oil and gas in the unlined rock caverns under the critical water sealing height is discussed. The validation of the suggested water sealing criterion is proved by the results of two-phase flow numerical analysis and in-situ measured data.