Abstract: Underground oil reserves are established near the sea, which can reduce the cost of oil transportation and have strong military significance. However, the proximity to the coastline makes it easy for seawater to intrude into the reservoir area, resulting in corrosion of the metal structure in the reservoir area, thus reducing the service life of the reservoir. A simulation study on the risk of seawater intrusion was carried out with an offshore underground oil reserve project as the background. A refined three dimensional hydrogeological model with 15 geological structures such as integrated faults, jointed dense zones and water conduction channels was constructed, and the groundwater seepage field and solute transport field in the reservoir area under the natural state and during the operation period were studied. The study shows that (1) there is no seawater intrusion in the undergroundoilreserve in the natural state. (2) During the operation period, a horizontal water curtain with a distance of 10 m and a diameter of 0.11 m was only set at 25 m above the top of the cavern reservoir, exceeding the cave by 50 m around, which could not effectively prevent seawater intrusion. (3) When the cave reservoir was running for 9 a, the Cl⁻ mole concentration on the southeast side of 3# cavern room exceeded 7 mol/m³, and seawater began to invade the cavern group; when it was running for 22 a to 23 a, the seawater invasion speed was the fastest; when it was running for 41 a, the Cl⁻ mole concentration on the southeast side of 3# cavern room exceeded 143 mol/m³, and the seawater invasion reached the strong corrosion degree of steel; when it was running for 50 a, all the cavern rooms except 1# cavern room were invaded by seawater, 3# and 4# cavity rooms have some areas to reach strong corrosion degree, seawater invasion is the most serious. It is suggested to set up additional facilities such as vertical water curtain to enhance the prevention and control of seawater intrusion. The research results can provide reference for the suppression of seawater intrusion in underground oil reserves in offshore areas.