In order to explore the adaptability of Polygonum capitatum to different understory light conditions, the photosynthetic characteristics and chlorophyll fluorescence parameters of P. capitatum grew in forest gap (FG), under forest canopy (FC) and in forest transition zone (FT) were examined, meanwhile, the response of photosynthetic capacity and chlorophyll fluorescence parameters to different photo intensities were analyzed. The results showed that the apparent quantum yield (AQY) of P. capitatum were higher in FG (0.028) and FC (0.023) than in FT (0.019). There was little difference in light compensation point among three habitats（35.00～66.77 μmol/（m2·s））. The light saturation point (LSP) and the maximum net photosynthetic rate (Pn) were both lower in FG (808.45 μmol/(m2·s), 6.24 μmol/(m2·s)) than those in FT (1 201.43 μmol/(m2·s), 8.86 μmol/(m2·s)) and FC (1 780.42 μmol/(m2·s), 13.72 μmol/(m2·s)), whereas dark respiration rate (Rd) was higher in FG (1.97 μmol/(m2·s)) than those in FT (0.80 μmol/(m2·s)) and FC (1.67 μmol/(m2·s)). The maximum photochemical efficiency of PSII (Fv/Fm) and potential PSII activity (Fv/F0) were both higher in FC (0.803, 4.08) than those in FG (0.801, 0.402) and FT (0.785, 3.65). The yield of photosystem II (φPSⅡ) and photochemical quenching (qP) decreased with increasing of light intensity, whereas the electron transport rate (ETR) and non-photochemical quenching (NPQ) increased. P. capitatum adapted weak light condition under forest by reducing dark respiration rate and improving the photochemical efficiency. At the same time, P. capitatum reduced the qP and increased NPQ to avoid light damage under the bright light conditions. These features of P. capitatum indicated that it has good light adaptability.