For plants, their leaf is an important organ to obtain resources. It is conducive to understanding the biological invasion mechanism by exploring the relationship among leaf functional traits and responses to environmental factors. In this study, we focused on Bidens alba, a species with three levels of invasion (mild, moderate, and severe) in urban plant communities and measured its 14 leaf functional traits and 5 environmental factors (light intensity, atmospheric humidity, and soil physical and chemical properties). Additionally, we analyzed the resource allocation pattern, absorption and utilization strategy in the process of invasion by one-way ANOVA, correlation analysis, redundancy analysis and other methods. The results showed that: (1) As the degree of invasion increased, several leaf traits also changed significantly. Specifically, leaf width, leaf area, leaf dry weight, leaf dry matter content and leaf total phosphorus content increased significantly with the increase of invasion degree. There were the highest specific leaf area and leaf total nitrogen content under moderate invasion. There were the lowest leaf thickness, leaf volume and fresh weight under moderate invasion. (2) There was close relationship among leaf functional traits. In mild invasion degree, the leaf length was observably correlated with leaf width and leaf area. Leaf fresh weight was observably correlated with leaf dry weight. There was significant correlation between leaf total nitrogen content and leaf total phosphorus content under moderate invasion. The specific leaf area was significantly correlated with leaf dry weight, leaf dry matter content, and leaf tissue density under severe invasion. (3) Redundancy analysis showed that light intensity and soil humidity were the main factors affecting the invasion of Bidens alba. In the process of invasion, the increase of light intensity and the decrease of soil humidity caused the increase of leaf dry matter content, leaf thickness, leaf tissue density and leaf total carbon, and the decrease of leaf area. These changes suggested a shift in the adaptation strategy of Bidens alba from resource acquisition in the early stages of invasion to resource conservation in the later stages. To sum up, under the increasing pressure of external conditions, Bidens alba tended to adopt different strategies at various stages of invasion. In the process of mild invasions to moderate invasions, Bidens alba tended to adopt acquisition strategy (low input-high return). But as the invasion progressed from moderate to severe, its strategy shifted to resource conservation (high input-low return).