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Photosynthetic measurements were taken in the water controllable field at Gucheng，Hebei Prvince when winter wheat was in the early milk stage， and different water controls were designed for no water stress（CK）， low water stress （soil moisture 50% to 70%） and high water stress （soil moisture under 50%）. Diurnal changes of photosynthetic parameters of flag leaves in winter wheat were measured by Licor6400 under CK and different water stress conditions in shining days.The main results are:（1） Diurnal change of the net photosynthetic rate of flag leaves showed a singlepeak type and there occurred no “noonbreak” under CK conditions. Contrarily， all the diurnal changes of the net photosynthetic rate of flag leaves under different water stress conditions showed the doublepeak type，and the noon break in winter wheat under the high water stress condition shortened for 2 hours compared with that under low water stress condition.（2） Diurnal changes of transpiration rate all showed the doublepeak type under three water controls，and there was obviously positive correlation between transpiration rate and stomatal conductance.The feedback sensitivity of flag leaves kept at a lowlevel when winter wheat was in the appropriate soil moisture （CK） and soil drought （soil moisture under 50%） conditions.（3） The net photosynthetic rate of flag leaves and the wateruse efficiency （WUE） under low water stress condition were over 2.8% to 9.0% and 10.6% to 12.9% higher than that under the CK condition respectively，which may explain the phenomena that the high yield of winter wheat can be gained at a water stress condition.