Forests are the main component of terrestrial ecosystems and play an important role in maintaining the dynamic balance of the biosphere and the atmosphere. It is of great significance to explore the change of vegetation index of forest ecosystem and its response to climate factors to optimize the ecological environment construction in China. In this study, based on MODIS-EVI (Enhanced Vegetation Index) data and meteorological data from 2001 to 2020, we investigated the changes of vegetation index in forest ecosystems and its response to climate factors during the growing season in China using trend analysis and partial correlation analysis. The results showed that the EVI of forests increased at a rate of 0.028/10a from 2001 to 2020, and the significantly improved area accounted for 39.63% of the total area. During the growing season, EVI was positively correlated with temperature, precipitation, global radiation and soil moisture, with soil moisture having the most significant effect, especially in the central and southern regions. The time-lag effect of EVI in response to various climatic factors varied among different regions. There was no time-lag effect in the response of EVI to temperature in all six regions. The lag in the response to global radiation was 3 months in all of them. In the northwestern region, the lag period of EVI response to precipitation was 3 months and 6 months in Southwest China. The lag period of EVI response to soil moisture was 6 months in East China. There was no time-lag effect in EVI response to precipitation and soil moisture in the rest of the regions. Overall, soil moisture was the main factor affecting vegetation changes in the growing season.