Biopolymers are eco-friendly materials used to enhance soil mechanical strength. However, due to the degradation effects of microbials, the strength of biopolymer-reinforced soil tends to attenuate over time. To address this issue, this study introduces another bio-based green material, biochar, to reinforce the soil in combination with the biopolymer. Static and dynamic triaxial tests were conducted to investigate the reinforcement effects. Water-soluble carbohydrates and β-glucosidase activity, along with soil’s internal friction angle and cohesion, were measured after curing samples for 1, 7, 14, 21, and 28 days to quantify biodegradation. A scanning electron microscopy (SEM) test was performed to illustrate microstructure changes due to biodegradation and to reveal how biochar mitigates the degradation process in biopolymer-treated soil. It was found that xanthan gum forms hydrogels through hydrolysis reactions and establishes hydrogel bonds among soil particles, enhancing the soil’s static and dynamic strength. However, microbes can degrade these hydrogels, resulting in a significant attenuation of soil reinforcement. Due to biochar’s porous morphology and hydrophilic functional groups, the free water in the soil is gradually released and participates in the hydrolysis of xanthan gum. Additionally, hydrogels are captured by the voids in the biochar, leading to stronger inter-particle binding. Consequently, the strength of XG-treated soil exhibits a trend of slow growth over time. Biochar’s angular structure enhances interlocking and embedding among particles, leading to a 42% increase in shear strength compared to untreated soil, and a 21% increase compared to soil treated with xanthan gum alone. Therefore, the combined approach of xanthan gum and biochar is advisable for reinforcing soil in engineering practice.