Abstract: Abstract: It is essential to understand the underlying physiological regulation of the effects of nitrogen fertilization to yield. Exploring the characteristics of flag leaf photosynthesis after the headed stage is of importance since the flag leaf assimilation product of this stage are the main sources of yield.In this research, winter wheat fields with sufficient drip irrigation and different nitrogen fertilization treatments were selected as the object of the study. Three nitrogen fertilization levels were designed. Net photosynthetic rate to photo active radiation (PAR) curves of different stages were measured with Li-cor 6400 portable photosynthesis system, to get the capacity of photosynthesis, i.e. the maximum photosynthetic rate (Amax) and the apparent quantum yield (α). The specific leaf mass (SLA), leaf nitrogen content per leaf mass (N-mass), leaf nitrogen content per leaf area (N-area) and 13C carbon isotope discrimination rate (Δ) were also determined by analyzing the leaf samples after photosynthesis measurements. Yield and water use efficiency were also estimated.The light curves were fit using a non-rectangular hyperbolic regression to obtain the relevant parameters for a comparison among treatments. Linear fitting was used for the light data at 0-150 μmol/(m2·s), the slope of the line was α and the intercept was the respiration rate (Rd).The results showed that the more nitrogen fertilization significantly increased the parameters of Amax and α (P=0.046), extended the duration of the better photosynthesis function, especially for the highest nitrogen treatments (N3, 221 kg/hm2). Rd and PARc showed no significant differences among treatments for the measurements on May 21st-23rd and June 2nd -3rd. The mean values of Rd of the three measurements were 1.54, 1.98 and 1.90 μmol/(m2·s), respectively. The Amax values gradually decreased over the duration of the field trial. The Amax of N3, N2 and N1 treatments on June 2nd-3rd were 14.0%, 28.9% and 39.9% lower than the Amax values measured on May 4th-5th, respectively. The Amax of the high N treatment, i.e. the N3 treatment, was kept fairly high even at the end of the milky stagy in the June 2-3 measurement. That's the reason of the highest yield of the N3 treatment. However, the evapotranspiration were not affected significantly by nitrogen treatments in this study. In this case, the water use efficiency of the N3 treatments was the highest. The yield of winter wheat for N3, N2 and N1 were 6646.5, 6 302.2 and 5810.2 kg/hm2, respectively. Y of N3 was 14.4% higher than that of N1 with significantly difference (P=0.046). The WUEa values of N3, N2 and N1 treatments were 14.34, 13.41 and 12.25 kg/hm2/mm respectively.The enhancement of Amax were related to the SLA, N-mass, N-area and Δ. Except that Amax was with marginally significant linear correlation with Δ (P=0.057) for N1 treatment, under all other treatments, there were significant linear relationships between Amax and SLA, Δ, N-mass and N-area. N treatments affected those relationships significantly. With the decreased N fertilization, absolute values of slopes of the regression lines were increased, suggesting that Amax is highly sensitive to the above parameters if lower N applied. In the relationships between Amax and leaf N content, the absolute value of Y-intercept of the N3 treatment was the highest and the slope was the lowest, suggesting that the Amax of N3 were higher at a given leaf N content, both at the mass and area basis. This may be interpreted as a result of changes in nitrogen partitioning in the photosynthetic apparatus.The results of this study explained the underlying photosynthetic regulation of yield enhancement by nitrogen fertilization in winter wheat fields. Moreover, the photosynthesis and leaf traits parameters determined in this study are readily to be used in some crop models to increase the accuracy of estimation under conditions of nitrogen treatments.