To clarify the spatiotemporal changes and distribution patterns of grassland fractional vegetation cover(FVC)in the Three-River-Source National Park(Tangbei Area)from 2000 to 2023,we quantified the dynamic variations in grassland vegetation under different FVC levels,and explored the correlation between vegetation changes and precipitation variations the sake of for grassland ecological conservation and sustainable development in the region.Using MODIS NDVI products as the data source and combining them with field measurements from established grassland plots,the corresponding NDVI values were extracted as model variables.Regression analysis was employed to fit ground-based measurements with remote sensing data,and the optimal equation was selected to invert FVC in the study area over the past two decades(2000-2023).The results indicate that:(1)High-FVC grassland vegetation is primarily distributed in the southeastern part of the study area.(2)Grassland FVC shows an overall improving trend.The proportion of high and medium-high FVC grassland areas increased by 65.14% and 15.77% respectively,while the proportion of medium,medium-low,and low FVC grassland areas decreased by 18.87%,26.24%,and 18.45% respectively.(3)Vegetation coverage is influenced to some extent by precipitation variations.The proportion of high and medium-high FVC grassland areas shows a positive correlation with annual precipitation(Pearson correlation coefficients of 0.37 and 0.28 respectively).In contrast,the proportion of medium,medium-low,and low FVC grassland areas exhibits a negative correlation with annual precipitation(Pearson correlation coefficients of 0.34,0.31,and 0.12 respectively).From 2000 to 2023,the ecological condition of grasslands in the Three-River-Source National Park(Tangbei Area)improved significantly.There was a substantial increase in high and medium-high FVC grassland areas and a reduction in medium-low and low FVC grassland areas,reflecting the positive effects of ecological protection and restoration projects.Additionally,vegetation changes exhibited a significant correlation with precipitation.Grasslands of different coverage levels,except for low-FVC areas,showed a sensitive response to increased precipitation.