Water deficit is a major limiting factor in vegetation recovery and reconstruction in the Tarim Basin in the extremely arid desert region of northwest China. Populus pruinosa Schrenk is an ecologically important species growing in this region. However, the P. pruinosa population has been declining in recent years because of the low underground water table, mainly caused by increased human water consumption. Currently, the mechanisms underlying the decline in photosynthesis of P. pruinosa under soil water declines are not clear. The objective of our study was to investigate the effects of the soil water gradient on photosynthesis, and the relationship between photosynthesis and soil water content. Our study will enhance our understanding of the photo-physiological characteristics of P. pruinosa exposed to soil water declines and provide valuable information for the protection of this vulnerable species. Saplings of P. pruinosa were planted in pots under four different soil water gradients. The effects of the soil water declines on the photosynthetic characteristics and stress resistance of P. pruinosa were evaluated by analyzing gas exchange, photosynthetic light and CO₂ response curves, pigments, stem water potential, and osmotic adjustment substances. The results showed that:(1) The net photosynthetic rate(P_n), transpiration rate, stomatal conductance, intercellular CO₂ concentration, and light use efficiency of P. pruinosa decreased with a gradual decrease in soil water content by 35.53%, 25.32%, 48.18%, 15.62%, and 40.92%, respectively, under severe soil drought in comparison to normal soil water content(CK). However, the phenomenon of "noon break" was apparent, and non-stomatal limitation was responsible for a reduction in P_n. Water use efficiency(WUE) could be improved by 3.05% under mild drought conditions relative to the CK, and P_n was maintained at a high level. (2) Light ecological amplitude decreased, CO₂ compensation point increased, ribulose 1,5-diphosphate (RuBP) regeneration was limited, and light/CO₂utilization efficiency, rubisco activity, and photosynthetic efficiency of P. pruinosa decreased with a decrease in soil water content. Compared to the CK, light response parameters(maximum net photosynthetic rate[P_nmax], apparent quantum efficiency[AQY], light saturation point[LSP], carboxylation efficiency[CE], Photosynthetic capacity[A_max], photorespiratory rate[R_p], maximum carboxylation rate[V_cmax], maximum electron transport rate[J_max], and triose-phosphate utilization rate[TPU]) significantly decreased(P < 0.05). Notably, P_nmax, A_max, and biochemical parameters of photosynthesis decreased by 42.65%, 38.26%, and 57.10% (moderate drought) and 63.01%, 65.88%, and 73.43% (severe drought), respectively. (3) Stem water potential and pigment content were significantly reduced(P < 0.01), the pigment composition proportion of the light reaction center was altered, and membrane lipid peroxidation(MDA) was significantly enhanced(P < 0.01) with decreasing soil water content. P. pruinosa mainly accumulated proline and soluble protein to eliminate reactive oxygen and alleviate the impairment of the photosynthetic apparatus under soil drought. Irreversible damage was caused to the photosynthetic system of P. pruinosa, and normal photosynthesis was seriously inhibited under severe drought. Therefore, declines in soil water content resulted in drought stress and reduced photosynthetic ability. This indicated that the decline in the P. pruinosa population might be caused by the lower soil moisture. In conclusion, the soil water content for maintaining higher P_n and WUE in P. pruinosa forestlands should be approximately 60%-65% of the field capacity in the Tarim arid-desert region to adhere to the management principles of efficient water conservation and vegetation restoration in extremely arid areas.