The effect of nitrogen fertilization on leaf chlorophyll fluorescence was studied in field-grown winter wheat during grain filling underrainfed conditions in Loess Plateau. Results showed that the actual photochemical efficiency of PSⅡ 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 and180 kg 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 comparedto those in the morning. In the afternoon, the F PSⅡ partialy or completely recovered to the levels in the morning. The values of F PS Ⅱin 0 and 90kgha-1 treatments recovered to 87.3 and 81.5% of those in the morning. In 180kgha-1 treatment, the F PSⅡ in the afternoonwas 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 thatapplication of nitrogen fertilizer could increase the light energy conversion efficiency, the potential activity of photosynthetic reactioncenter, and the non-photochemical dissipation of excess light energy, which can prevent leaf photosynthetic apparatus from damage ofenvironmental stress. However, there was no significant difference in the values of F PSⅡbetween 90 and 180kgha-1 nitrogentreatments, indicating that the excess nitrogen was unfavorable to photosynthesis. The effect of nitrogen fertilization on leaf chlorophyll fluorescence was studied in field-grown winter wheat during grain filling under strain conditions in Loess Plateau. Results showed that the actual photochemical efficiency of PSⅡ 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 180 kg ha-1 nitrogen treatments at noon were 0.197, 0.279 and 0.283, respectively, which decreased 57.7, 56.4 and 40.2% as compared to those in the morning In the afternoon, the F PSII partialy or completely recovered to the levels in the morning. The values ​​of PSIIin 0 and 90kgha-1 treatments recovered to 87.3 and 81.5% of those in the morning. In 180kgha-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 These results indicate that application of nitrogen fertilizer could increase the light energy conversion efficiency, the potential activity of photosynthetic reactioncenter, and the non-photochemical discharge 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 PSⅡbetween 90 and 180kgha-1 nitrogentreatments, indicating that the excess nitrogen was unfavorable to photosynthesis.