Changes in the vegetation net primary productivity (NPP) are primarily influenced by climate change and human activities. However, the dominant factors and their contributions at different time stages remain unclear. Here, we developed a method to quantify the relative contributions of climate and human factors to the vegetation NPP in China from 1982 to 2020 using three types of NPP components: the actual NPP (aNPP), potential NPP (pNPP), and human-occupied NPP (hNPP). The results showed that: (1) From 1982 to 2000, the annual average aNPP in China was 501.0 g C/m2, with a total annual aNPP of 378.2 Tg C. From 2001 to 2020, the annual average aNPP increased to 517.1 g C/m2, with a total annual aNPP of 391.1 Tg C. (2) From 1982 to 2000, human activities primarily drove aNPP changes, with contribution rates of 34.26% and 50.43% for increases and decreases, respectively. However, from 2001 to 2020, climate played a larger role in aNPP increases (the contribution was 43.90%), while human impact dominated decreases (the contribution was 83.75%). This study demonstrated that climate has played an increasingly important role in NPP increase over the past 40 years and will provide scientific strategies for climate adaptation.