As an invasive species, Eupatorium adenophorum threatens the biodiversity security in Southwest China. To explore its light adaptation characteristics and related eco-physiological mechanism, this paper studied the physiological and morphological properties, e. g., gas exchange, fluorescence kinetics, chlorophyll content and specific leaf weight (SLW), of E. adenophorum grown in Xishuangbanna, China, during the dry season under four light regimes (100%, 36%, 12.5% and 4.5% full sunshine). Under full sunshine, the Fv/Fm and phiPS II decreased with increasing diurnal light intensity, but quickly re-increased when light intensity decreased, indicating that photoinhibition was occurred but not serious. E. adenophorum under full sunshine could increase excessive light energy dissipation through the enhancement of diurnal thermal dissipation and anti-oxidation and the reversible inactivation of PS II reaction center, increase light energy utilization, and decrease light absorption by increasing SLW and decreasing chlorophyll concentration. All these processes enabled E. adenophorum to avoid the photodamage of photosynthetic apparatus. Under low light intensities (36%, 12.5% and 4.5% of full sunshine), E. adenophorum had a higher light absorption and utilization efficiency through decreasing SLW and diurnal thermal dissipation, increasing chlorophyll concentration, and keeping high photosynthetic capacity. The effective dissipation of excessive light energy under high light and the sufficient absorption of light energy under low light allowed E. adenophorum to utilize light energy effectively and grow healthily in a wide range of light intensity, which might be one of the reasons that made E. adenophorum a vigorous invader.