Grassland coverage dynamics and its relationship to climatic factors at different spatial and temporal scales in the Three Rivers Source Region(TRSR)from 1982 to 2012 were analyzed based on MODIS-NDVI and GIMMS-NDVI data and climate data.The responses of the grassland coverage to climatic variations at annual and monthly time scales were analyzed.The results showed that grassland coverage distribution had increased from northwest to southeast across the TRSR during the study period.During 1982-2012,the mean grassland coverage of the TRSR was 48.73% and exhibited spatial heterogeneity,being the highest(65.45%)in the source region of Yellow River and the lowest(4.25%)in the source region of Yangtze River.The mean grassland coverages of meadow,alpine and sub-alpine meadow,plain grassland,alpine and sub-alpine grassland, and desert grassland were 59.86%,57.39%,39.50%,33.70%,and 14.13%,respectively.The grassland coverage of the study area increased with a linear tendency of 0.23%per year,of which the source region of Yellow River presented the fastest increasing trend(0.27%per year).The grassland coverages in meadow,alpine and sub-alpine meadow,and plain grassland were mainly affected by temperature,whereas grassland coverages in desert grassland and alpine and sub-alpine grassland were affected more by precipitation.At the monthly timescale, grassland coverage had a one-month time-lag effect compared to temperature.However,there was less than one month or no time-lag effect for precipitation.After the Three Rivers Nature Reserve was founded,the growth of grassland coverage accelerated to some extent.Meanwhile,the sensitivity effect of temperature on grassland coverage increased throughout the entire district.In addition,the influence of precipitation on grassland coverage decreased in the source region of Yellow River.The ecological engineering and grassland protection measures have gained some achievements in the TRSR,whereas a falling trend of grassland coverage was exacerbated in some areas,especially desert grassland;therefore,greater attention should be paid to this region.