For a general study of the daily variation regulation on the photosynthetic parameters and its relation with the main impact factors about relative humidity RH, atmospheric CO₂ concentration C_a, atomspheric temperature T_a, leaf temperature T_l, photosynthetic active radiation PAR, vapor pressure deficit on leaf VPD of Pueraria wallichii DC., photosynthesis and water physiological characters of P. wallichii DC. on the Ordos plateau during the growth season were measured using LI-6400 portable photosynthesis system. The results showed that the net photosynthetic rate P_n of P. wallichii DC. had a diurnal change with two peaks at 12:00 and 16:00, exhibiting an obviously phenomena of photosynthetic midday depression. The transpiration rate T_r took on a curve with only one peak, which reached the highest value at 16:00. The water use efficiency WUE had a diurnal change with two peaks and it was higher in the morning than in the afternoon. The responsiveness of P_n to PAR, T_a and RH was different and significant. This result shows that there is the complicated relation among the parameters of coefficient, with increase of net photosynthetic rate, increase of transpiration rate, increase of stomatal conductance, decrease of intercellular CO₂ concentration, and the decline of red deer spend P_n is not mainly affected by stomatal factors.