Abstract: In order to study the photosynthesis of Acer saccharum Marsh, and provide the theory reference to the induction of Acer saccharum Marsh. The author tested photosynthetic indexes of the Acer saccharum Marsh by using Li-6400 photosynthesis analyzer system in three different seasons, and analyzed net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), stomata conductance (Gs) and transpiration rate. The results showed that: the curves of net photosynthetic rate (Pn) and transpiration rate (Tr) had two peaks during these seasons, and had midday depression. Compared the three seasons of Acer saccharum Marsh daily variation of net photosynthetic rate and transpiration rate, the curves of water use efficiency had two peaks during these seasons, and the water use efficiency in autumn was higher than in spring and summer. The author also analyzed the relationship between Pn and multivariate. Different seasons had different Pn main influencing factors. The main factors effecting on Pn were photosynthetic active radiation and conductance of stomata.