Abstract: Luxury transpiration water consumption has a lower contribution to the formation of photosynthetic products and crop yield, and the flowering and filling stage is the key period for the yield of winter wheat. In order to efficiently and accurately regulate the transpiration of crops, to determine the soil moisture threshold affecting luxury transpiration is important. In this study, the winter wheat variety 'Shixin 828' was used as study material. Pot experiments were carried out in an artificial climate growth box. The effects of soil moisture on stomatal conductance, photosynthetic rate and transpiration rate of crops were quantitatively studied in order to define soil moisture threshold for luxury transpiration at flowering and filling stage. The results showed that the stomatal conductance was closely related to soil water suction. When soil water suction was low, stomatal conductance decreased rapidly with the increase in soil water suction. When soil water suction was high, stomatal conductance decreased slowly. When water suction was less than 1.2 MPa, the photosynthetic rate was close to the maximum value. Then as water suction increased, the photosynthetic rate decreased gradually in the form of a parabola. The transpiration rate decreased linearly with a reduction rate of 2.3 mmol·m^-2·s^-1·MPa^-1 as soil water suction increased. The coupling relationship between photosynthesis and transpiration was described by the Michaelis-Menten equation. When the transpiration rate was lower than K_Tr=2.179 mmol·m^-2·s^-1, the photosynthetic rate increased linearly with the transpiration rate; but when the transpiration rate was higher than K_Tr, the increase in net photosynthetic rate became slower, and luxury transpiration occurred. When soil water suction corresponded to the luxury transpiration threshold of K_Tr (1.76 MPa), the leaf photosynthetic rate was at a high level (about 16 μmol·m^-2·s^-1), and the leaf water use efficiency (WUE_L) was at the highest level7.3 μmol (CO₂)·mmol^-1(H₂O). In summary, the luxury transpiration of wheat leaf started with a shift from the highest WUE_L to lower while photosynthetic rate maintaining a rather higher level. Based on the Michaelis-Menten equation, the relationship between photosynthesis and transpiration, and the linear relationship between transpiration and soil water suction, soil water suction of 1.76 MPa was found to be the soil water threshold for the start point of luxury transpiration of wheat leaves during the flowering and filling stage.