The effect of the degradation of sulfur and phosphorus type additives on the extreme pressure and antiwear properties of gear oil was studied by a Falex four ball tester.The infrared absorption peaks of the typical functional groups in the sulfur and phosphorus type additives were obtained by Fourier transform infrared spectrometer.The degradation law of extreme pressure antiwear additives in a gear oil of Harmony Locomotive during the running test was studied.The results show that the extreme pressure properties is decreased and antiwear properties is increased of the gear oil during the running test.The degradation of extreme pressure and antiwear additives occurs in the mileage of 100 000 km,and the degradation is mainly the phosphorus type additives with the P=S groups,which is up to 17% at the end of the test. The test oil could meet the requirement of the extreme pressure and antiwear properties of the harmony locomotive.