Abstract: Effect of the water on crop was mainly photosynthesis and respiration，finally determining the yield of the crop．In the North China Plain，the crop is often in the condition of the lack of water．This causes the contradiction between the photosynthesis and respiration．By measuring the parameters of the exchange of water and gas through the leaf，we got the relationships among the water use efficiency，photosynthesis rate，respiration rate，stomatal conductance and leaf temperature．The water use efficiency，photosynthesis rate and respiration rate of the three kinds of water treatments presented a likely diurnal change，but the water use efficiency of the water deficit was larger than the others．When the photosynthes is rate was between 20 and 26umol／m²·s，the water use efficiency increased fast．And when the respiration rate was between 5 and 7umol／m²·s，the water use efficiency increased fast．The water use efficiency was sensitive to leaf temperature．And it was suitable for the increase of the water use efficiency when leaf temperature was between 40C and 42℃ ．And the water use efficiency presented an ascensive trend with the increase of the stomatal conductance And it was the most suitable for the increase of water use efficiency when the stomatal conductance was between 140 and 200umol／m²·s．