Patented and equipped with a pressure relief valve, which integrates the functions of oil injection, oil storage, waterproof, oil level display and pressure release. When the internal oil quantity of the transformer is too low or the pressure is too high, the oil can be manually injected and released by the oil level gauge. When the pressure drops to the closing pressure value of the valve, the valve will be reliably closed to protect the fully sealed transformer from the outside world, to ensure the safety and reliability of the transformer operation.