An underwater oil storage method, which includes the steps of: a) excavating a cellar in the ground earth and constructing a wall structure in the cellar to form a water pool, b) setting an oil tank in the water pool, c) supplying a predetermined amount of water to the water pool, and d) filling the oil to be stored into the oil tank and regulating the amount of water in the water pool subject to the amount of storage oil stored in the oil tank for enabling water in the water pool to support the oil tank and storage oil in the oil tank and to keep the oil tank in balance by means of the physical characteristic that the specific gravity of the storage oil is lower than water.