Abstract: The photosynthetic characters of Acer truncatum and Robinia pseudoacacia under doubled CO2 concentration were investigated. The results showed that the average photosynthetic rate of A. truncatum and R. pseudoacacia grew under doubled CO2 concentration after 60 d and 90 d were 117.81% and 106. 11% of those grown under normal CO2 concentration, respectively. Under doubled CO2 concentration, Fo was more than the contrast, and Fm, Fv were lower than the contrast in August and September. However, this was the opposite in the October and November. The Fv/Fm and Fv/Fo of PSⅡ times were 0. 008, 0. 012 times and 0. 230, 0. 201 times respectively. Their transpiration rate decreased 26.82% and 42.30% under doubled CO2 concentration. The diurnal curve of transpiration rate was convex in doubled CO2 concentration and was double peak curves in contrast.