The temperate grassland is the sensitive to climate change and is the ecologically fragile area of the terrestrial ecosystem in China, and it is an important part of the grassland resources. Due to global warming, the extreme climate impact brought by climate change is getting bigger, so exploring the temporal and spatial changes of vegetation coverage in the temperate grasslands in China and the effects of changes in water and heat conditions on vegetation growth are of greatly practical significance to the environmental protection of regional ecology, the sustainable development of animal husbandry, and more rational and effective use of grassland resources. Based on the long-term meteorological data (precipitation, temperature and radiation) and GIMMS NDVI 3g data from 1982—2015, this study used trend analysis and correlation analysis to explore the changes of grassland NDVI (Normalized difference vegetation index) in the growing season in the temperate regions of China, and the pattern of climate change, as well as the simultaneous and lagging effects of hydrothermal conditions on the NDVI. The results showed that: (1) From 1982 to 2015, the average temperature and total solar radiation in China's temperate grassland growing season showed an increasing trend, the total amount of precipitation was a downward trend, and the temperate grassland climate gradually became "warm and dry". (2) From 1982 to 2015, the inter-annual variation of NDVI in the growing season of temperate grassland in China was dominated by precipitation factors. Especially after 1999, the influence of precipitation on the inter-annual variation of NDVI in the growing season of temperate grassland in China was more significant. (3) From 1982 to 2015, the NDVI of China's temperate grassland growing season was with an increasing trend, and it was positively correlated with water and heat factors. The significant increase in thermal conditions (temperature and solar radiation) during the growing season controlled the growth trend of the NDVI. (4) From 1982 to 2015, the NDVI of China's temperate grassland growing season had an obvious lagging response to the inter-annual variation of water and heat factor. The NDVI responded most obviously to the temperature lag in the early stage of the growing season, and precipitation became the main controlling factor for the lagging influence on vegetation growth in the middle and late growing seasons.