Abstract: The photosynthetic characteristics and main influential factors of leaves in Alnus cremastogyme seedlings were systematically studied by using the Li-6400 portable photosynthesis system in this paper. The main results were as follows: (1)The net photosynthetic rate (Pn) of leaves has obvious diurnal and seasonal change. The diurnal change of Pn in the middle period (July) showed the double-peak curve, and the peak value occurred at 9:00 and 15:00 or so, and 13.73 μmol/m²·s and 16.03 μmol/m²·s respectively. However, the daily change of Pn during the beginning period(April) and the late period(Octorber) had only one peak value respectively.The order of Pn at different position's leaves was middle leaves > upper leaves > lower leaves. The seasonal change of Pn at middle leaves was double-peak curve type, and the first peak value of which was 19.3 μmol/m²·s in June and the second 18 95 μ·mol/m²·s in August. The upper and the lower had one peak value. (2) The light compensate point (LCP) of leaves was about between 24.36 μmol/m²·s and 52.16 μmol/m²·s, and the light saturate point (LSP) above 1 000 μmol/m²·s, and the apparent quantum efficiency between 0.029 5 and 0.049 4.These showed that the leaves of A.cremastogyme had the higher utilization efficiency on weak light and strong light. (3) The CO₂ compensate concentration of photosynthesis was between 60.86 μmol/mol and 84.73 μmol/mol, and the CO₂ saturate concentration 800 μmol/mol or so, the carboxying efficiency between 0.032 19 and 0.047 76, which indicated that A.cremastogyme belonged to the C 3 type plant.(4) By the relative analysis and stepwise multiple regression, it was found that Pn had the obvious relationship with Cond, Ci, Trmmol, Tair, Tleaf, RH, and PAR.