Abstract: The photosynthetic properties of two-year-old seedlings of Alnus rubra were observed under natural conditions by means of Li-6400 Photosynthesis Analyzer to explore the influence of meteorological factors upon the seedlings' photosynthesis.The results indicated that there was difference in diurnal variation of the net photosynthetic rate,i.e.,the curve of diurnal variation of the net photosynthetic rate determined on April 24^th,October 10^th and November 20^th was characterized by single-peak,while the curve of diurnal variation of the net photosynthetic rate determined on Junuary11^th and August 9^th was characterized by double-peak.The diurnal variation of transpiration course of all the dates determined was one-peak curve.By means of correlation analysis and stepwise multi-regression analysis,the correlation between net photosynthetic rate and transpiration rate,the correlation between net photosynthetic rate and meteorological factors including quanta flux density,CO₂ concentration,air temperature and the relative humidity of atmosphere was analyzed,and the regressive equations were formulated.