With the increasing concern over the potential environmental pollution caused by traditional inorganic chemical stabilizers (such as fly ash and cement), this study investigates the improvement effect and micro-mechanism of guar gum and Palm fibers, two environmentally friendly and renewable materials, on expansive soil. Through disintegration tests, unconfined compressive strength tests, triaxial compression tests, and SEM analysis, the study examines the enhancement of the mechanical properties of expansive soil by guar gum-Palm fiber composite materials. The results show that the guar gum-Palm fiber composite significantly improves the compressive and shear strength of expansive soil. The optimal ratio is 2% guar gum, 0.4% Palm fiber, and 6mm Palm fiber length. When the Palm fiber content is fixed, increasing the fiber length initially increases and then decreases the unconfined compressive strength of the composite-improved soil. The addition of guar gum increased the unconfined compressive strength by 187.18%, and further improved it by 20.9% with the addition of Palm fibers. When the fiber length is fixed, increasing the Palm fiber content first increases and then levels off the peak principal stress and shear strength (including cohesion and internal friction angle), improving by 27.30%, 52.1%, and 12.4%, respectively, compared to soil improved with only guar gum. Micro-analysis indicates that guar gum enhances the bonding strength between soil particles by forming a gel matrix, thus improving the water stability and mechanical properties of the soil, while Palm fibers inhibit crack propagation through reinforcement. The synergistic effect of both significantly enhances the performance of the composite-improved soil, offering good economic and environmental advantages, providing new insights for expansive soil engineering management.