Gross primary production (GPP) has increased substantially across the China's Loess Plateau (LP) over the past two decades, a trend widely attributed to vegetation greening driven by land use and land cover change (LUCC), particularly the Grain to Green Program (GGP). However, most existing studies rely on statistical or machine learning approaches, limiting mechanistic understanding. Here, we combined a process-based ecosystem model with Shapley value attribution and multi-source observations to quantify the drivers of GPP dynamics in the LP from 2001 to 2019. We found that 84% of the LP exhibited significant greening without LUCC, indicating that climate and other environmental changes-rather than LUCC or the GGP alone-dominanted regional greening. The combined effects of climate change and CO2 fertilization explained similar to 79% of the GPP increase, through both direct enhancement of photosynthetic rate (similar to 1.28 Tg C yr(-2)) and indirect increase in leaf area index (similar to 5.07 Tg C yr(-2)). Notably, in contrast to previous studies, we detected a small but spatially widespread negative impact of TEMP on GPP, whereas declining vapor pressure deficit exerted a positive influence. The GGP contributed similar to 11.3% of the GPP increase (similar to 0.95 Tg C yr(-2)), primarily by altering vegetation structure across similar to 9.5% of the LP's area, while other LUCC events accounting for the remaining similar to 9.7% of the GPP increase. These findings offer new insights into the relative roles of climatic and anthropogenic drivers of ecosystem productivity and provide mechanistic evidence to inform future ecological restoration and land management strategies.