Abstract: Information on the relationship between crop photosynthetic rate and water use efficiency at the leaf level and these at the canopy level is important on the study of theory and practice of high efficiency and water-saving agriculture. The authors simultaneous made instantaneous CO₂ and H₂O exchange rate, and instantaneous water use efficiency measurement at both leaf and canopy levels in a rain-fed cropping system with spring corn during filling stage by Li-6400 infrared gas analysis meter and eddy correlation system. The results showed that in arid areas water stress was the main factor influencing the net photosynthesis rate and water use efficiency. In the diurnal course, when soil relative moisture was about 40% at 0～100 cm soil layer, the maximum of net photosynthetic rate at leaf level occurred at about 10∶00 under soil water stress, and at about 12∶00 at canopy level on sunny days, the maximum leaf photosynthetic rate reached 1.3 mg·m^－2·s^－1, almost close to those of irrigation corn at similar latitude area, and the maximum canopy assimilation rate were 0.9 mg·m^－2·s^－1 , reached 54.5% of those of irrigation corn at similar latitude area during filling stage, respectivelyl; The maximum values of instantaneous water use efficiency at leaf and canopy level were 0.16 g(CO₂)/g(H₂O) and 0.06 g(CO₂)/g(H₂O), and ranged between 0.0055 g(CO₂)/g(H₂O) and 0.0123 g(CO₂)/g(H₂O), 0.0113 g(CO₂)/g(H₂O) and 0.0197 g(CO₂)/g(H₂O) around noon, respectively. Instantaneous net photosynthetic rate and water use efficiency at leaf level were higher than those at canopy level after 10∶00.