Carbon cycling of terrestrial ecosystem has become the research front and focus of global change and geo-science. Precision evaluation of the carbon storage and carbon sink plays an important role in the assessment of future atmospheric C02 concentration, climate change prediction and its impact on terrestrial ecosystem. Existing studies have mainly focused on the amount estimation of carbon storage and carbon sink of regional ecosystem, but analysis of the change process on time scale and the spatial difference of changing trend is limited. This paper uses MODIS GPP data, together with land use data and soil organic carbon density data to analyze the spatial-temporal change of carbon storage of grassland ecosystem in the Three-River Headwaters Region from 2000 to 2010. Meanwhile, the spatial-temporal change of carbon sink is analyzed using MODIS GPP, China FLUX and America Flux data applied in the modeling of grassland ecosystem respiration to specify the carbon-carrying capacity and its change process of the study area and provide scientific basis for protection and management of regional grassland ecosystem. The results show that: (1) the total carbon storage of grassland ecosystem is 53.38*10~8 t with the average carbon density of 14.94 kg·m~(-2) (measured in C). The carbon storage of soil and vegetation is 53.07*10~8t and 0.31*10~8t, respectively, with the average carbon density of 14.85 kg·m~(-2) and 86.77 g·m~(-2), respectively; (2) during recent 10 years, the total carbon sink of grassland ecosystem is 0.4*10~8t with the average carbon sink per area of 86.80 g·m~(-2)·a~(-1) (measured in C), which shows that the grassland ecosystem in the study area is a carbon sink; (3) both of the total amount of carbon storage and carbon sink of grassland ecosystem show an increasing trend since 2000; (4) the distribution of carbon storage and carbon sink of grassland ecosystem and their changing trend shows great spatial difference; (5) It is proved that the MODIS GPP/NPP data can be applied to analyze the carbon storage and carbon sink pattern, as well as its changing trend of grassland ecosystem in large scale, which is more efficient and convenient than traditional approach.