To reveal the effects of groundwater depth change on gas exchange and chlorophyll fluorescence of Populus euphratica grown in arid region, we measured and conducted comparative studies among the daily course of gas exchange, light response curve, photosynthetic response to CO₂ (PN-C_i curve), chlorophyll fluorescence parameters, leaf water potential, and chlorophyll content of P. euphratica grown at different groundwater depths (4.91m, 6.93m and 8.44m) in the lower reaches of Tarim River. The results showed that the net photosynthetic rates (P_N) measured at 10:00, the minimal fluorescence (F₀), the maximal fluorescence (F_m), the actual photochemical efficiency of PSⅡ (Ω_PSⅡ), the apparent rate of electron transport (ETR), the non-photochemical quenching coefficient (NPQ) and the leaf water potential measured at noon (Ψ_midday) were obviously changed with the groundwater depth increasing from 4.91m to 6.93m and 8.44m. Among them NPQ increased by 109% 127%, suggesting that P. euphratica could increase the thermal dissipation of excess excitation energy by NPQ process to avoid suffering the damage of photosynthetic apparatus. However, Ω_PSⅡ decreased by 24% 29%, indicating that the increasing groundwater depth resulted in the photochemical change of light-adapted leaves. Moreover, F₀ decreased by 12.2% 18%, F_m decreased by 7% 15.5%,ETR decreased by 17% 22%, and Ψ_midday decreased by 31.6% 45.6% respectively, showing that the degree of drought stress from which P. eupuratica suffered would increase when the groundwater depth increased from 4.91m to 6.93m and 8.44m. Additionally, when the groundwater depth increased from 6.93m to 8.44m, the F_m, F_V/F₀ and F₀ measured at predawn and noon did not change obviously, showing that P. euphratica could hold the PSⅡ photochemical activity and the PSⅡ reaction center was not damaged under this condition. Similarly, Ψ_midday, Ω_PSⅡ and ETR also did not change significantly, showing that trees grown at groundwater depths between 6.93m and 8.44m likely suffered from the same degree of drought stress. When the groundwater depth ranged from 4.91m to 8.44m, the apparent quanta efficiency (φ) changed slightly, with minimal damage to photosynthetic structures; and the maximum velocity of Rubisco for carboxylation (V_cmax), the maximum potential rate of electron transport (J) and the triose phosphate utilization rate (TPU) did not change markedly, suggesting that p. euphtatica can keep the activity of electron transport on a certain extent and the output quantity of triose phosphate from leaves; simultaneously, the photochemical quenching (qP) changed insignificantly with the increasing groundwater depths, indicating that the opening degree of PSⅡ reaction center of P. euphratica grown at different groundwater depths was approximate the same. Moreover, the maximal photochemical efficiency of PSⅡ (F_v/F_m) measured at predawn and noon possessed the higher values ranging from 0.85 to 0.87 and from 0.81 to 0.82, respectively, indicating that PSⅡ original photochemical efficiency was not influenced and the photosynthetic structures were not damaged by the increasing drought stress. Overall, the drought stress degree P. eupuratica suffered from within the groundwater depths 4.91 8.44 m was not beyond the tolerance of trees, that is, the photosynthetic capability of P. eupuratica did not get an irreversible harm even if the groundwater depth increased to 8.44m.