Abstract: Diurnal changes of photosynthesis of winter wheat during grain filling stage under sprinkler and surface irrigation conditions were studied in sunny days to investigate the differences among photosynthesis features of different irrigation methods. Photosynthesis features including net photosynthesis rate, transpiration rate, stomatal conductance, intercellular CO₂ concentration of winter wheat flag leaves were measured using a portable LI-6400 photosynthesis system under sprinkler and surface irrigation conditions. The results were as follows: compared with that under surface irrigation condition, the net photosynthesis rate of winter wheat flag leaves under sprinkler irrigation was increased, and the intercellular CO₂ concentration was reduced. The transpiration rate and stomatal conductance of winter wheat flag leaf under sprinkler irrigation condition were higher than those under surface irrigation condition at photosynthetic midday depression time, but were lower at other diurnal time(12∶00～14∶00). The photosynthetic midday depression under sprinkler and surface irrigation conditions was resulted from the stomatal limitation factor and nonstomatal limitation factor. Under sprinkler irrigation condition, it was predominantly due to the stomatal limitation factor caused by decreased photosynthetic capacity of mesophyll cells, while it was predominantly due to stomatal limitation under surface irrigation condition. The results indicated that the photosynthesis activity of mesophyll cells in winter wheat flag leaves under sprinkler irrigation was improved, which made the limiting factor of photosynthesis rate change from nonstomatal limitation to stomatal limitation gradually.