<p>Loess slopes are prone to erosion, underscoring the importance of exploring eco-friendly materials for slope protection purposes. In this study, lignin and guar gum-modified loess samples are used as research objects, and their comprehensive performance and slope protection effects are evaluated via direct shear, disintegration, permeability, scanning electron microscopy (SEM), and slope model tests. The results show that lignin enhances the shear strength of loess by increasing its internal friction angle, whereas guar gum does so by improving the degree of cohesion. Both lignin and guar gum can reduce the disintegration rate and permeability level. The overall improvements exhibited by the properties of loess modified by 2% lignin and loess modified by 4% guar gum are optimal. SEM reveals that lignin forms filamentous networks that wrap and connect soil particles, and guar gum generates gel-like substances that fill pore spaces; both pathways enhance the performance of loess. Lignin and guar gum have synergistic effects on loess, but their proper contents and ratios are critical. Otherwise, the lignin may penetrate the guar gum aggregates and weaken the overall binding effect. Loess modified by a 1:3 lignin-guar gum composite with 4% content maximizes this synergy. Slope model tests reveal that lignin maintains the integrity of slopes, guar gum significantly reduces slope erosion effects, and their combination leverages component advantages to minimize crack formation and erosion. This study clarifies the improvement mechanism exhibited by eco-friendly material-modified loess, providing a reference for achieving slope protection in loess regions.</p>