Background and Aims <p>While lignin is known to enhance soil quality, its specific contribution to the mechanical reinforcement of rooted soil remains less explored. This study aims to elucidate the synergistic effect of lignin on root development and soil aggregate formation in loess, and to determine its consequent impact on the shear strength of rooted loess.</p> Methods <p>The aggregate stability tests, root morphology investigation, and direct shear tests were carried out on the samples of bare soil, alfalfa (<i>Medicago sativa</i> L.) rooted soil, and ryegrass (<i>Lolium perenne</i> L.) rooted soil added with different mass percentages of lignin (0%, 0.2%, 0.3%, 0.4%, and 0.5%).</p> Results <p>Lignin transformed micro-aggregates into larger aggregates. The maximum increase rates of mean weight diameter (MWD) and water-stable aggregates content &gt; 0.25 mm (R_0.25) were up to 101.22%, 50.14% (Alfalfa) and 101.13%, 64.52% (Ryegrass) resulted from the lignin addition. Lignin significantly reduced the soil erodibility factor (K) and percentage of aggregates destroyed (PAD), and enhanced the soil aggregate stability. The maximum shear strength and the optimal plant morphology occurred at 0.4% (bare soil), 0.4% (alfalfa), and 0.3% (ryegrass) lignin additions. Redundancy analysis found that plant root length contributed the most to shear performance. Lignin enhanced soil shear strength by increasing soil cohesion and promoting root growth.</p> Conclusions <p>0.3%-0.4% lignin addition was the optimal amount to promote the formation of soil aggregate structure and accelerate plant growth in the loess, thus contributing to the improvement of shear resistance of the rooted soil.</p>