To explore the improvement effect and reinforcement mechanism of Xanthan gum on completely weathered residual gypsum breccia remolded soil, liquid-plastic limit and direct shear tests are conducted to examine variations in consistency limits and strength indices of soil samples with varying Xanthan gum contents, thereby determining the optimal mixing ratio. Based on triaxial and scanning electron microscopy tests, the mechanical properties and microstructural characteristics of Xanthan gum-modified soil samples at the optimal mixing ratio are further examined, elucidating the reinforcement mechanism. The results reveal the following: (1) The completely weathered residual gypsum breccia remolded soil exhibits a layered structure with face-to-face aggregated connections, displaying distinct fissures and voids. Xanthan gum interacts with fine soil particles and moisture, generating bonding, cementation, filling, and film-forming effects that tightly connect soil particles. These interactions form irregular crystals that fill pores, remarkably enhancing interparticle bonding strength and soil mechanical properties. (2) The optimal Xanthan gum content is 1.5%, at which modified soil samples demonstrate increased elastic limit stress and peak stress (an 8.0% enhancement), pronounced strain-hardening characteristics, and a uniformly slow increase in pore water pressure with axial strain. The peak pore water pressure notably decreases, benefiting surrounding rock stability. (3) Modified soil samples show an 11.4%－16.2% increase in effective peak stress, a 61.8% increase in effective cohesion, a 12.9% increase in the total stress internal friction angle, and a 15.5% improvement in the average elastic modulus, confirming considerable enhancements in both strength and stiffness.