Alhagi sparsifolia is a dominant plant species in the Manas River Basin on the northern piedmont of the Tianshan Mountains in Xinjiang. Based on soil temperature monitoring data from the basin, temperature conditions associated with freeze–thaw processes were established at -10, -5, -1, 0, and 5 °C, with 25 °C used as an above-freezing reference condition. Independent root samples were used for single-root tensile tests under each target temperature condition. Root chemical composition analysis and ESEM observations under selected temperature conditions were also conducted to compare root tensile properties among different temperature states. The results show that root diameter and maximum tensile force exhibit significant positive power-law relationships under all temperature conditions, and the effect of root diameter on maximum tensile force varies among diameter classes. The unit root diameter tensile-force increment of coarse roots (D4–D8) is generally higher than that of fine roots (D1–D3). Comparisons among temperature groups show that the absolute differences in maximum tensile force are generally greater for coarse roots than for fine roots. The lowest mean maximum tensile force of each D4–D8 diameter class occurs in the 5 °C group, whereas the temperature ranges corresponding to the largest differences among D1–D3 classes vary with root diameter. Root stiffness, strength, and deformation capacity also show certain differences among different target temperature states. Cellulose, hemicellulose, and lignin are the major cell-wall chemical components of Alhagi sparsifolia roots, and their contents exhibit different variation patterns with root diameter. Overall, root diameter is an important morphological factor affecting the tensile-bearing capacity of Alhagi sparsifolia roots, and root tensile properties differ among temperature states associated with freeze–thaw processes. These findings provide references for the selection of root mechanical parameters and ecological protection assessment of freeze–thaw slopes in cold and arid regions.