With the Li-6400 system,we measured the diurnal change of photosynthetic rate(P_n),stomatal conductance (G_s),intercellular CO₂ concentration(C_i),transpiration rate(T_r),water use efficiency(WUE) and other physiological indicators of Populus euphratica and P.pruinosa in the middle reaches of Tarim River.The purpose was to explore the physiological characteristics of adaptation to the arid environment and for further study the photosynthetic water physiological characteristics of P.euphratica and P.pruinosa as well as,to protect P.euphratica and P.pruinosa of Tarim basin and to provide research material and theoretical basis.The following results have been obtained:(1)In the entire process of the diurnal variation of photosynthesis,average of P_n,G_s,C_i,T_r and WUE of P.euphratica were higher than those of P.pruinosa.(2)The diurnal variation of photosynthetic rate of P.euphratica and P.pruinosa were double peak curve,and there are the phenomenon that “photosynthetic sleep”.(3)At noon,the photosynthetic rate of P.euphratica,P.pruinosa were decreased that the impact by stomatal limitation.In the evening,the photosynthetic rate of P.euphratica,P.pruinosa were decreased that the impact by the weakened light intensity,the greater impact by non-stomatal limitation.(4)Determine the photosynthetic rate changes of P.euphratica,P.pruinosa,the environmental factors as follows:pho-tosynthetic photon flux density (PPFD)>stomatal conductance (G_s)>air humidity (RH)>air temperature(T_air);decide the transpiration rate of P.euphratica,the environmental factors as follows:T_air>PPFD>G_s>RH;decide the diurnal variation transpiration rate of P.pruinosa the environmental factors as follows:PPFD>T_air>G_s>RH.The P.euphratica and P.pruinosa to adapt to the environment of the arid and semi-arid regions in the long process of adaptation to form different physiological and ecological strategy.P.euphratica improved WUE and took the initiative to adjust stomatic conductance to decrease transpiration,while P.pruinosa reduced the hurt of arid environment and drought with high transpiration and low WUE.