This study investigated Tamarix ramosissima growing on coppice dunes in the southwestern edge of the Gurbantunggut Desert. We employed a spatial sequence approach in place of a temporal sequence to analyze antioxidant enzymes and osmotic regulators in the leaves of T. ramosissima in different developmental stages of coppice dunes, providing a theoretical foundation for understanding the survival mechanisms of native plants in arid desert regions. The results showed that: (1) Soil moisture content exhibited a declining trend from the initial stage to the decline stage of coppice dunes. T. ramosissima in the declining stage of coppice dunes displayed significantly higher leaf catalase (CAT) activity, alongside malondialdehyde (MDA), soluble protein (SP), proline (PRO), and abscisic acid (ABA) content compared to other developmental stages of coppice dunes, indicating that the plants in the declining of coppice dunes experienced severe stress conditions and an increasing trend in membrane lipid peroxidation damage. The superoxide dismutase (SOD) activity in leaves in the growth and stable stages of coppice dunes was significantly higher than that in the other stages of coppice dunes, suggesting enhanced antioxidant capacity of T. ramosissima in these coppice dunes. (2) Compared to spring, the activities of peroxidase (POD) and ascorbate peroxidase (APX) in T. ramosissima leaves on coppice dunes significantly decreased in summer and autumn, while the contents of osmotic adjustment substances significantly increased. In contrast, the ABA content decreased markedly in summer, indicating that T. ramosissima preferentially maintains cellular water balance through osmotic adjustment and reduces its reliance on ABA-mediated stomatal closure under arid and high-temperature conditions. (3) Correlation analysis and structural equation modeling revealed that leaf temperature, and photosynthetically active radiation (PAR), and potential evaporation promoted the activity of the antioxidant enzyme system and the accumulation of osmotic adjustment substances in T. ramosissima on the initial stage and growing stage of coppice dunes. Increased leaf temperature not only boosted the activity of leaf antioxidant enzymes but also raised the risk of cellular damage from membrane lipid peroxidation. Enhanced photosynthetically active radiation promoted the accumulation of endogenous abscisic acid (ABA). Soil moisture content and leaf temperature were key environmental factors regulating antioxidant enzyme activity and oxidative damage indicators in plants in the stable stage and declining stage of coppice dunes. Decreased soil moisture content significantly increased the activity of antioxidant enzymes and the content of osmotic adjustment substances, indicating that T. ramosissima maintains cellular water balance by enhancing antioxidant enzyme activity (to scavenge reactive oxygen species, ROS) and synthesizing osmotic adjustment substances under drought stress. Comprehensive analysis demonstrates that T. ramosissima can adapt to extremely arid environments by increasing leaf antioxidant enzyme activity, enhancing osmotic adjustment capacity, and regulating endogenous hormone responses under the development of coppice dunes.