Abstract: Through artificial shading, this paper studied the phenotypic plasticity of invasive species Ambrosia trifida in its morphology, biomass allocation, and photosynthesis characteristics in response to different light intensities. As compared with the control, shading increased the stem height, crown width, leaf area, specific leaf area, and the proportion of leaf biomass in total biomass of A. trifida significantly, but decreased the total biomass, biomass per unit leaf area, and root to shoot ratio. Under natural light condition, the crown width and leaf area were smaller and the root to shoot ratio was larger, which benefited the decrease of water loss under high temperature and high light intensity and manifested the stronger phenotypic plasticity of A. trifida in its morphology and biomass allocation in response to different light intensities. Under shading, the mean daily net photosynthetic rate, transpiration rate, and stomatal conductance decreased, while the stomatal CO₂ concentration increased. At noon when the light intensity was the highest, the photosynthetic rate, transpiration rate, and stomatal conductance under low shading reached the maximum. Under moderate shading and high shading, the chlorophyll content increased significantly, and the chlorophyll a/b had a significant decrease, which could improve the utilization of lightenergy by A. trifida under shading environment.