Grassland ecosystem, one of the key component of terrestrial ecosystems, performs critical functions in climate regulation, soil and water conservation, windbreak and sand fixation, and the maintenance of biological diversity. The Qinghai-Tibetan Plateau (QTP), also known as the "Third Pole" of the Earth, is the largest and highest plateau in the world, with an average altitude of about 4000 m. As the important ecological security barrier in China, the QTP is regard as a sensitive and fragile area under global climate change. In recent years, the regional climate and environment of the QTP have changed dramatically due to intensified climate change and human activities, such as global warming, water pollution, grassland degradation and desertification, which have seriously hindered the sustainable development of local society and economy. Alpine grassland is the dominant plant functional type on the QTP and its vegetation dynamics in response to climate change and intensified human activities has been widely concerned. Normalized difference vegetation index (NDVI) has been widely applied in the study of vegetation dynamics because it can effectively reflect the vegetation coverage and growth status. Temperature and precipitation are considered to be the primary climatic factors affecting vegetation dynamics, while grazing intensity and human population are considered as the dominant anthropogenic factors. Therefore, understanding the mechanisms of the responses of alpine grassland to climate change and human activities is of great significance to predict the future of grassland under these disturbances. In this study, we analyzed the spatio-temporal changes of the NDVI of alpine grassland, temperature, precipitation, human population and grazing intensity by trend analysis from 1982 to 2013 on the county-scale. Besides, we used the panel data model to investigate the effect of climate changes and human activities on the NDVI in 143 counties of the QTP in the past 32 years. The results showed that: (1) the NDVI increased generally, and the plant growth increased in most of the areas but decreased in some specific areas. (2) there was an increasing tendency for the mean temperature (i.e., warmer) and total precipitation (i.e., more humid) over the growing season. (3) in the long-term, climate factors dominated the change of NDVI in alpine grassland on the QTP. The increase of precipitation and temperature increased the NDVI, whereas the continuously increasing grazing intensity decreased the NDVI in alpine grassland.