The invention discloses a constant-pressure gas storage water pumping compressed gas energy storage system and an energy storage method. A high-pressure constant-pressure gas storage cabin, a water-gas jointly containing cabin and a water storage pool are arranged, stable sealing pressure is provided through the water-level difference of the high-position water storage pool and the low-position high-pressure constant-pressure gas storage cabin, the investment cost of the system is greatly reduced, the recovering age limit of the energy storage system is shortened, the operating economy of thesystem is improved, the energy loss in the throttling pressure stabilizing process is avoided, it is guaranteed that a compressor and an expansion machine run under the constant working condition, thesystem efficiency is improved, the water-gas jointly containing cabin and the water storage pool form a water pumping energy storage system, the high-pressure constant-pressure gas storage cabin anda water storage pool close to the ground form a water pumping energy storage system, that is the double water pumping energy storage systems are formed, in the energy storage process and the energy releasing process, flowing changing speeds of water in the water storage pool and water in the low-position high-pressure gas storage cabin are slow, the loss of the water in the flowing process is relatively small, and the efficiency of the water pumping energy storage system formed by the high-pressure constant-pressure gas storage cabin and the water storage pool close to the ground is improved.