Abstract: This paper analyzed the effect of Regulated Deficit Irrigation(RDI) on photosynthesis，transpiration，stomatal conductance， the ratio of dry root weight to the matter weight above ground， yield and consumptive use of water as well as water use efficiency of maize in the field. It reveals the mechanism of yield improved using the optimum RDI regime by which the change of stomatal behavior can improve the ratio 　photosynthesis and transpiration. The experiment result demonstrated that the moderate water stress in seedling (soil moisture content is 50 %～60 % of field capacity) and the light water stress in jointing stage (soil moisture content is 60 %～70 % of field capacity) is considered as the optimum irrigation schedule in order to obtain high yield and high water use efficiency. 