Abstract: The photosynthetic characteristics of Populus euphratica in four forest ages (2, 3, 5 and 7 years) were studied in order to reveal the photosynthetic physiological characteristics of Populus euphratica in different forest ages and provide basis for regeneration and rejuvenation of Populus euphratica in the lower reaches of Tarim River. Li-6400xt portable photosynthetic instrument was used to measure the physiological factors such as net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r) and ecological factors such as temperature (T), photosynthetic effective radiation (PAR) and relative humidity (RH). Correlation analysis, stepwise regression analysis and path analysis were used to systematically study P_n of Populus euphratica at different ages in fast-growing period (June, July and August) and the characteristics of daily change and its relationship with physiological and ecological factors. The results showed that the P_n was a single peak curve in the fast-growing period, and there was no significant "lunch break" phenomenon. The P_n, T_r and WUE of 7-year-old Populus euphratica were significantly higher than those of the 3 other ages (P < 0.05). The P_n of fast-growing Populus euphratica was positively correlated with PAR and G_s (P < 0.01), the direct effect and decisive coefficient of PAR were large, the direct effect of G_s was greater than that of other factors, the determination coefficient was next to PAR, and other factors also have different effects on P_n. The responses of Populus euphratica to ecological and physiological factors were different. T is the limiting variable of P_n of 7-year-old Populus euphratica in June, T_l is the limiting variable of P_n of 2-year-old and 7-year-old Populus euphraticain August, and all physiological and ecological factors promote P_n in July. The results showed that P_n of Populus euphratica at different forest ages was mainly affected by PAR and G_s, but there was no simple linear relationship, which was the result of a comprehensive effect of many factors. With the increase of forest age, the photosynthetic performance and productivity of Populus euphratica were better.