Abstract: Loess slopes are highly prone to erosion and failure under extreme rainfall due to their loose structure and high permeability, posing significant risks to engineering safety and ecological stability. This study investigates a synergistic modification strategy using Sesbania gum and CaCl₂-(NH₄)₂CO₃ to enhance the impermeability and erosion resistance of loess. Through permeability testing and simulated rainfall erosion experiments, we demonstrate that the in situ formation of CaCO₃ combined with Sesbania gum hydrogel markedly improves the mechanical and hydraulic properties of loess. The experimental results demonstrate that the optimal modification effect on loess was achieved with 10% sesbania gum and 7.5% calcium chloride/ammonium carbonate(1:1 molar ratio). After 28 days of curing, the modified loess exhibited approximately two orders of magnitude reduction in permeability coefficient and a remarkable 181.9% increase in surface hardness. Despite microcrack formation during repeated permeability cycles, the modified loess maintained low permeability and structural integrity, exhibiting remarkable self-healing capacity. Under extreme rainfall simulation, slope failure time increased dramatically from 74 minutes(unmodified)to 925 minutes(modified), and soil loss was significantly reduced. SEM analysis revealed that the improvements arise from the cohesive and pore-filling effects of Sesbania gum and CaCO₃, which bind soil particles and densify the structure. This study provides both experimental validation and theoretical support for the application of biopolymer-CaCO₃ composite technology in loess slope reinforcement.