Abstract: To explore the invasion mechanisms of Conyza canadensis and develop effective control measures, this paper studied the photosynthetic characteristics of the invasive plant and its main associated plants Ixeris chinensis and Commelina communis. The light saturation point and light compensation point of C. canadensis were 1634.00 and 23.84 μmol·m^-2·s^-1, respectively, which were between those of the two associated plants. The maximum net photosynthetic rate of C. canadensis below light saturation point was 28.12 μmol·m^-2·s^-1, being much higher than that of the two associated plants. The apparent quantum yield of C. canadensis was 0.06, equal to that of I. chinensis  but higher than that of C. communis. The CO₂ saturation point and CO₂ compensation point of C. canadensis were 834.00 and 23.69 μmol·mol^-1, respectively. The maximum net photosynthetic rate of C. canadensis below CO₂ saturation point  was 31.97 μmol·m^-2·s^-1, which was between that of the two associated species. The carboxylation efficiency of C. canadensis was 0.078, being higher than that of the two associated species. The variations of photosynthetically active radiation and CO₂ concentration had little effects on the stomatal conductance and transpiration rate of the three plants, but significantly affected their water use efficiency. C. canadensis had higher photosynthetic rate and material accumulation capability, and its high productivity could be one of the important factors for its successful invasion.