In order to investigate the photosynthetic characteristics of Onobrychis viciaefolia seedlings and the main environmental factors directly influencing photosynthesis of the seedlings,the photosynthetic characteristics of Onobrychis viciaefolia seedlings and the diurnal variations of the factors were measured using Li-6400 photosynthesis system by a pot experiment.Meanwhile,the effects of the factors on photosynthetic rate(P_n) were analyzed using correlation analysis and stepwise multi-regression analysis.Results showed that the diurnal variation of the P_n presented a feature of single peak curve with its peak value appearing at 12:00 am,about 27.63 μmol/(m²·s).The daily average P_n over the observation time was calculated to be 22.33 μmol/(m²·s).Besides,according to correlation analysis,photosynthetically active radiation(PAR) was found to be the main environmental factor directly influencing the daily P_n of Onobrychis viciaefolia seedlings and the PAR had a highly significant positive correlation with the P_n.