In order to better simulate the aging of the fireresistant oils of steam turbines in service,a hydraulic bench was set up.By simulating the actual operation status of fireresistant oils in steam turbine governing system,the aging test of three commonly used phosphate ester fireresistant oils(A1, A2, A3)in power plants was carried out.The results show that the acid value change of A1 fireresistant oil is the slowest,while the acid values of A2 and A3 fireresistant oils are increased greatly,and the acid value of A3 fireresistant oil exceeds the standard of running oil at the end of the test.A3 fireresistant oil has the best air release performance.The volume resistivity of A1 volume resistivity maintains a normal range,and the amount of sludge precipitation is the least.Comprehensive analysis shows that the aging resistance of A1 fireresistant oil is better than that of A2 and A3 fireresistant oils.