Abstract: Daily variations in net photosynthetic rate of wheat flag leaf under different N, P and K rates were observed by LI-6400. Four treatments — high N (300 kg·hm^-2) and K (75 kg·hm^-2) with low P (32.5 kg·hm^-2) (N4P2K2), high K without N and P (N1P1K2), high P (65 kg·hm^-2) without N and K (N1P3K1) and high N and P without K (N4P3K1) were used in the experiment. The results revealed differences among daily variations in net photosynthetic rate (P_n) of wheat flag leaf under different fertilization treatments. N4P2K2 treatment significantly promoted wheat flag leaf P_n. However, N4P3K1 treatment did not significantly promote wheat flag leaf P_n. Photosynthesis was generally weak in N1P1K2 and N1P3K1 treatments. Daily variations in flag leaf P_n under the same treatment but at grain filling, milky and mature stages were also compared. Difference in P_n among three main growth stages was highest under N1P3K1 treatment and lowest under N4P3K1 treatment, it was medium under N4P2K2 and N1P1K2 treatments. P_n was relatively high under N1P1K2 treatment at mature stage. This suggested that K fertilizer influenced daily variations in photosynthetic characteristics of wheat flag leaf at mature stage.