Abstract: Influence of environmental factors on photosynthetic physiologic indexes and water use efficiency of winter wheat leaves was studied in semi-arid rain feed agriculture region in Loess Plateau. Diurnal fluctuation of physiological rate(Pn) of leaf was a not obvious two-hump curve and diurnal fluctuation of transpiration rate(Tr) was a typical resupinate “U” and the Pn and Tr in a.m. were distinctly higher than in p.m.,but the time at which the maxium appeared was different in different growth stage.The time of maxium appeared of Tr and Pn was same in elongation stage and the time of maxium appeared of Tr was later than that of Pn in grain filling stage.The infuence of environmental factors on physiologic indexes in elongation stage was more obvious than that in grain filling stage. The most important environmental influencing factors on Pn and Tr were photosynthetic active radiation(PAR) and CO2 concentration.the influence of Temperature (T) and relative humidity(RH) on stomatal conductance(Gs) was progressively increased and the influence on Intercellular CO2 concentration was obvious.The diurnal fluctuation of water use efficiency was a not obvious two-hump curve and the time of maxium appeared was earlier than those of Pn and Tr.The daily mean WUE in filling stage was lower than that in elongation stage to 30.5%.The correlations among Pn,Tr and Cond were very significant and there was a significant negative correlation between temperature of leaf and Gs.The correlation inconsistency between temperature of leaf and Pn, Tr revealed that the wheat had some adaptability to dry because of the deficiency of precipitation in loess plateau and this is a opportunity in cultivating wheat of high WUE.