Abstract: Acid value enhances and oil quality worsens in the high-pressure fire -resistant oil used in the No. 7 steam turbine of a power plant. Organic impurities such as trimethylamine were found in the oil samples. After checking the anti-fuel oil system, it was found that there was gas leakage in the main valve area of the turbine. The oil temperature of the oil pipe and actuator of some high-pressure regulator in the anti-fuel system is high. Through a comprehensive analysis of multiple factors such as temperature changes and regeneration effects of high-pressure fire-resistant fuel, it is concluded that the partial overheating of the oil passage in the fire-resistant fuel system and failure of the regeneration device are the main causes of oil quality deterioration. In order to effectively control the oil quality and ensure the safe operation of the unit, targeted improvement measures have been proposed.