The invention discloses a seepage-preventing and seawater desalination combined ocean reservoir which comprises a water area separation system and a seawater desalination system. The water area separation system comprises a bulwark and a seepage-preventing curtain. The bulwark is used for separating inner and outer water areas originally communicating with each other intermittently into an open sea area and a fresh water reservoir. The seepage-preventing curtain is arranged at the bottom of the bulwark. The seawater desalination system is used for supplementing and replacing the water body inthe fresh water reservoir and comprises a seawater desalination film, a high-pressure saline area, a desalination water area and a high-pressure water pump. One side of the desalination water area communicates with the high-pressure saline area, the other side of the desalination water area communicates with the fresh water reservoir, and the seawater desalination film is arranged at the communication position of the high-pressure saline area and the desalination water area. By means of the seepage-preventing and seawater desalination combined ocean reservoir, a coast lagoon is used for building the ocean reservoir, ocean reservoir fresh water resources can be obtained, and a basic foundation is provided for long-term development and utilization of the coast lagoon and living needs of personnel on the coast lagoon.