Increased nitrogen deposition significantly impacts invasive plants, leading to population differentiation due to different environmental pressures during expansion. However, various populations respond differently to elevated nitrogen levels. This study explores the responses of central and edge populations of the annual invasive plant Galinsoga quadriradiata to different levels of nitrogen addition. The results indicate that the central population has a stronger need for nitrogen, with nitrogen addition promoting the growth of its aboveground parts, reducing intraspecific competition, and increasing reproductive allocation and total biomass. Specifically, nitrogen addition provides more nutritional resources, easing resource competition among plants, reducing intraspecific competitive pressure, and allowing plants to allocate more energy to growth and reproduction, thereby enhancing their expansion potential. In contrast, the edge populations respond differently to nitrogen. Although nitrogen addition promotes the growth of their underground parts and enhances root development, the impact on aboveground parts is smaller. The enhancement of underground parts helps edge populations better adapt to barren environments, improving their survival and competitive ability in new environments, thus increasing their expansion potential. Overall, the growth impact on edge populations due to nitrogen addition is smaller, possibly indicating they have exceeded their nitrogen limit. The study demonstrates that the degree of population differentiation in invasive plants at different invasion stages is a critical factor in studying their spread potential, aiding in predicting plant invasion trends under climate change and providing theoretical support for formulating targeted management strategies.