In order to monitor the running state of the gearbox lubricating oil of 5 fans of a wind field,the indexes affecting the decay of the gear oil and the service life of the gearbox,such as viscosity,viscositytemperature performance,moisture,acid value and pollutants of the gear oil,were monitored,and the changes of each index of the used oil under different operation times were analyzed.The results show that the kinematic viscosity of each gear oil has a certain change under different operating time,but no obvious decay occurs,which will not have a great impact on the gearbox.The test results of acid value and moisture also show that the gear oil has not been significantly deteriorated.The dynamic viscosity of gear oil has little change under different operating time,which indicates that the gear oil has not been seriously sheared and its structure has not changed significantly.Temperature has an important influence on gear oil,with the decrease of temperature,the dynamic viscosity of gear oil gradually becomes worse.The test results of gear oil contamination show that the contamination of gear oils has exceeded the standard,with the maximum of 22/20/17 and the minimum of 18/16/11.The pollution degree of gear oil of the gearbox in the wind field can be controlled by replacing the filter or installing the bypass filtration system to extend the service life of gearbox.