【Objective】 To demonstrate the variation characteristics of natural grass yellowing period in alpinegrasslands and its relationship with meteorological factors in Xinhai County,Three ‐ river source area from 1999 to2020. The aim was to provide a basis for the rational utilization and ecological environment protection of forage resourcesin Xinghai County and similar areas.【Metod】Utilizing phenological period data of herbage in alpine grasslandsand contemporaneous meteorological data observed by Xinghai County Meteorological Bureau in the Sanjiangyuanregion from 1999 to 2020, this paper analyzed the change trend of the yellowing period of alpine grasslands.Correlation analysis was employed to examine the relationship between the yellow period of herbage and climatic factorsduring the early period. Additionally a forecast model of the yellowing period was constructed using the regressionanalysis method.【Result】 (1) the average yellowing date of northwest stipa grass was September 4, with a differenceof 22 days between the earliest and the latest diurnal order.(2) From 1999 to 2020, the yellowing period ofherbage in Xinghai county had notably advanced in the recent 22 years.(3) Regression analysis indicated that the yellowingstage of herbage was primarily limited by temperature.【Conclusion】 The prediction model of the yellow periodcould reflect the rule of the yellowing time and its association with heat factors of Stipa grass.