Abstract: The use of xanthan gum biopolymer, which can suppress the expansion of cracks of in expansive soil without affecting the growth of slope vegetation, is a new approach for ecological protection of expansive soil slopes. The indoor wet dry cycle tests are conducted on the xanthan gum-modified expansive soil, and the digital image processing technology is used to quantitatively analyze the entire process of crack development. The influences of xanthan gum content on the characteristic indices of crack morphology in expansive soil are explored, and the changes in microstructure and mineral composition of expansive soil before and after improvement are analyzed through the scanning electron microscopy and X-ray diffraction experiments, The improvement mechanism of xanthan gum on expansive soil is explored based on the results of the expansion rate tests. The research results indicate that the addition of xanthan gum can effectively improve the water retention and crack resistance of expansive soil, and the average water loss rate and crack rate of the sample decrease with the increase of dosage. The Xanthan gum can reduce the impact of wetting-drying cycle on expansive soil, control the crack width within a small range, and make it easy to close during the humidification process. It mainly inhibits the cracking of expansive soil through two aspects. On the one hand, it forms a "bridging" structure with soil particles through bonding and cementation, enhancing the overall tensile strength of the soil. On the other hand, by filling and film-forming, it blocks the contact between water and soil particles, reducing the thickness of the hydration film of clay minerals.