The effect of nitrogen fertilization on leaf chlorophyll fluorescence was studied in field-grown winter wheat during grain filling under rainfed conditions in Loess Plateau. Results showed that the actual photochemical efficiency of PS Ⅱ reaction center (F PSⅡ) decreased significantly as leaf water stress progressed, however, the F PS was increased by nitrogen fertilization. The F PSⅡ of 0, 90 and 180kg ha-1 nitrogen treatments at noon were 0.197, 0.279 and 0.283, respectively, which decreased by 57.7, 56.4 and 40.2% as compared to those in the morning. In the afternoon, the F PS Ⅱ partialy or completely recovered to the levels in the morning. The values of F PS Ⅱ in 0 and 90kg ha-1 treatments recovered to 87.3 and 81.5% of those in the morning. In 180kg ha-1 treatment, the F PSⅡ in the afternoon was even higher than that in the morning. Application of nitrogen fertilizer significantly increased maximum photochemical efficiency (Fv/Fm), photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (qNP). These results indicated that application of nitrogen fertilizer could increase the light energy conversion efficiency, the potential activity of photosynthetic reaction center, and the non-photochemical dissipation of excess light energy, which can prevent leaf photosynthetic apparatus from damage of environmental stress. However, there was no significant difference in the values of F PS Ⅱ between 90 and 180kg ha-1 nitrogen treatments, indicating that the excess nitrogen was unfavorable to photosynthesis.