With field experiment, this paper studied the effects of nitrogen topdressing at different growth stage on the chlorophyll fluorescence and photosynthetic rate of winter wheat flag leaves, and on the grain yield of the wheat. Compared with that at getting-up and flagging stages, nitrogen topdressing at jointing stage improved the PSII (Fv/Fo) activity, maximum photochemical efficiency (Fv/Fm,), PSII coefficient of photochemical fluorescence quenching (qP), actual quantum yield of PSII electron transformation (phiPS II), and photosynthetic rate of flag leaves, and made the dissipation of non-radiant energy lower at early and middle filling stage, and higher at late filling stage, which made it possible for flag leaves to absorb more light energy for photosynthesis at early and middle filling stage, and reduce the light inhibition extent and senescence at late filling stage. Nitrogen topdressing at jointing stag increased the spikes and grain weight per kernel, resulting in the highest grain yield.