Abstract: In order to study photosynthetic characteristics of Chinese endemic single species Brachystachyum densiflorum (Rendle) Keng in summer and its influential factors, LCpro + SD automatic photosynthetic apparatus was used to test net photosynthesis (Pn) and multivariate step- wise analysis method was used to establish the relationship model between Pn and physiological and ecological factors. The results showed that daily variation of Pnof Brachystachyum densiflorum (Rendle) Keng in summer was like a curve of“twin peaks”, known as typical“noon break”; the light complementary point (LCP) and light saturated point(LSP) were 40.90μmol/(m²·s) and 979.77μmol/(m²·s), respecitvely, and apparent quantum yields (AQY) was 0.016. The influential physiological and ecological factors to affect Pn of Brachystachyum densiflorum (Rendle) Keng in summer mainly involve transpiration rate (Tr), intercellular CO₂ concentration (Ci) and photosynthetic active radiation (PAR). The research showed that Brachystachyum densiflorum (Rendle) Keng was characterized by higher LCP and lower LSP and AQY, indicating that its light adaptability was narrow, with low light energy utilization rate under weak light.