Abstract: In order to study the changes of characteristics and yield of spring wheat photosynthetic under drought after flowering， the changes of several photosynthetic index of spring wheat leaves under different water stress were measured. The results showed that the transpiration rate， stomatal conductance， intercellular CO 2 concentration and water use efficiency of flag leaf changed significantly as water stress increasing， so that the photosynthetic rate decreased by 37.67%-86.45%. The decline of Pn under serious and moderate water stress was mainly due to non-stomatal factors and under light water stress it was because of the decline of stomatal conductance， which caused the wheat premature senescence and influenced the grain filling， so that the yield decreased by10.70%-21.20%