[Objective] The spatiotemporal variation characteristics of vegetation net primary productivity (NPP) and their influencing factors were explored in Altay region of Xinjiang, China, and the adjacent area of Mongolia to provide a reference for cross-border regional ecological protection and sustainable development. [Methods] Regression analysis was used to reveal the spatiotemporal changes in vegetation NPP in Altay region adjacent to China and Mongolia, while correlation analysis and geographical detectors were applied to explore the main influencing factors of vegetation NPP in the cross-border area. [Results] ① From 2001 to 2021, the vegetation NPP in Altay region of Xinjiang, China, showed a slight decreasing trend of 5.49 g/(m²·10 a) (calculated by carbon), while the vegetation NPP in the western part of Mongolia showed a significant increasing trend of 27.06 g/(m²·10 a, p<0.05). ② Temperature (0.49, p<0.05) and precipitation (0.31, p<0.05) were the main factors affecting vegetation NPP in Altay region of Xinjiang, China, and the western part of Mongolia, respectively. ③ The interaction between temperature and land use type had the strongest explanatory power for vegetation NPP in Altay region of Xinjiang, China (0.68, p<0.05), while the interaction between precipitation and land use type had the strongest explanatory power for vegetation NPP in the western part of Mongolia (0.37, p<0.05). [Conclusion] From 2001 to 2021, the vegetation NPP in Altay region of China had been declining yearly, while the vegetation NPP in the western part of Mongolia had been increasing yearly. The interactive drivers of temperature, precipitation, and land use types are the main influences on vegetation NPP in both the Altay region of China and the western part of Mongolia.