Abstract: Water culture experiment was conducted using wheat (Triticum aestivum) as an indicating crop to study the influence of N level on photosynthesis rate , NR activity and content of RNA and DNA at the stage of reproductive growth. Four N levels were used as treatments: ① ников solution，② 1/5 N (16.8 mg N/L) Пряищников solution, ③ 1/2 N (42 mg N/L) Пряищников solution, ④ Пряищников solution (84 mg N/L) (CK). Each treatment had 5 replicates. During the experiment, the solution was aerated for 2-3 h everyday, the lost water of each pot was supplemented every day. The photosynthesis rate was measured in vivo with flag leaves. Take the flag leaf and the leaf next to it as material, the nitrate reductase activity (NRA), the content of nitrate, ammonium ions and chlorophyll, as well as RNA and DNA level were measured while the plants in other pots continued to grow until harvesting for measuring the biomass. The results showed as follows: with the increasing of exogenous N level, the leaf area, the photosynthetic rate and the content of chlorophyll were all enhanced; there were positive correlations between exogenous N level and activity of NR and the content of endogenous NH+4 nitrogen. It had the remarkable impact on the biomass and yield of wheat when supplying the different nitrogen level in the stage of reproduction.