Time lag and cumulative effects are crucial to the spatio-temporal dynamics of vegetation NDVI, and an in-depth understanding of their mechanisms is of great significance for the management and sustainable development of terrestrial ecosystems. Based on MODIS NDVI remote sensing data and meteorological data, this paper used Theil-Sen trend analysis and Mann-Kendall test to explore the temporal and spatial variation characteristics of vegetation NDVI in China from 2001 to 2020. Furthermore, by introducing the time lag and cumulative effect of climate factors, combined with partial correlation analysis and improved residual analysis, the comprehensive driving mechanism of climate change and human activities on vegetation dynamics was comprehensively evaluated, and the time response characteristics of climate factors and their effects on vegetation change were highlighted. The results showed that: (1) From 2001 to 2020, the NDVI of vegetation in China showed a fluctuating upward trend, and the spatial distribution showed that the NDVI in the eastern part was higher than that in the western part, and the NDVI in the northeast, north and south-central regions increased significantly, while the NDVI in the northwest and alpine regions was lower. In the future, about 13.84% of the region will continue to improve, and 26.15% of the region may be degraded. (2) The response of NDVI to temperature and precipitation has significant time lag and cumulative effect. The optimal time lag of temperature and precipitation was 0.199 and 0.201 months, respectively, and the cumulative time was 1.152 and 1.483 months, respectively. (3) 79.41% of the regional NDVI was significantly positively correlated with temperature, and 64.88% was significantly positively correlated with precipitation. The long-term impact of temperature was greater than that of precipitation. (4) In the NDVI growth area, 80.68% was driven by climate change and human activities, while in the NDVI reduction area, 81.60% was mainly driven by human activities alone, indicating that human activities played an important role in vegetation change.