Abstract: Background To address the prominent issues of loose structure, weak water retention capacity, and susceptibility to water erosion in sandy soil in arid regions; this study focused on sandy soil from typical wind-sand areas in China, and explored the improvement effect of bentonite-cotton straw fiber composite on its physical properties and its significance for soil and water conservation. Methods Through setting different ratios of bentonite and cotton straw fibers in the experiment, key indicators including capillary water holding capacity, total porosity, saturated water content, bulk density, water retention capacity, and hardness of the improved soil were measured. The water retention performance, structural change characteristics, and stability under dry-wet cycles were analyzed.Results 1) With the same addition ratio of bentonite, soil bulk density decreased by 14.66%-17.62% as the amount of cotton straw fiber increased, while capillary water holding capacity, total porosity, and saturated water content increased by 20.96%-24.58%, 5%-8.01%, and 25.72%-27.50% respectively. 2) Cotton straw fiber could effectively improve the physical properties of sandy soil and enhance its water retention capacity, with the optimal water retention effect achieved when the addition ratio of fine cotton straw fiber was 1.2% and 1.5%. 3) Bentonite could improve soil structural stability by enhancing particle bonding, increasing soil hardness by 476.13%, but after the second dry-wet cycle, soil hardness decreased significantly with a maximum reduction of 84.16%. 4) The results of scanning electron microscopy (SEM) revealed that bentonite could enhance the compactness of aeolian sandy soil by coating the soil particles and filling the gaps, and the addition of straw could further optimize the structure.Conclusions The study indicated that the composite of bentonite and cotton straw fiber may improve the physical structure of sandy soil, significantly enhance its water retention and erosion resistance, and has good ecological applicability and engineering potential, providing technical support and theoretical basis for slope stability, soil and water conservation, and ecological restoration projects in arid regions.