The phosphorus additive in the gear oils of wind turbine mainly plays the role of extreme pressure and antiwear.The trend of phosphorus content directly reflects the aging degree of gear oils.The Spectro oil spectrum analyzer was used to measure the content of phosphorus additive in the used gear oils.The monitoring limited value of phosphorus content was established by using probability and statistics theory and the regression equation of phosphorus content.The monitoring limited value of phosphorus content established based on a large amount of samples can be used to determine the aging degree of gear oils,which also provides a reference for the gear oil replacement of wind turbine.