Biopolymers are widely used as eco-friendly soil additives for soil stabilization. The combined use of biopolymers offers potential for enhancing soil performance, but research on their mixed effects on plant growth and soil mechanics is limited. This study utilizes xanthan gum and guar gum to prepare composite gum. Experiments on plant growth, mechanical properties test, and scanning electron microscopy (SEM) are conducted to explore the impact of the composite gum on the plant growth and mechanical properties of clay, as well as its ability to stabilize soil in conjunction with root systems. The experimental results reveal that optimal dosages of xanthan gum (2%), guar gum (1.5%), and composite gum (1%) significantly improve the planting performance of clay, while excessive dosages inhibit the effects. Composite gum demonstrates superior performance in enhancing clay shear strength by increasing cohesion. For substrates with roots, 1% composite gum achieves the best synergy with plant roots, increasing cohesive strength by 255.2% and shear strength by 70.2% compared to pure clay. Compared with xanthan gum and guar gum, composite gum improves shear strength by 11% and 2.2%, respectively. SEM analysis shows that the incorporation of biopolymers significantly enhances mechanical properties of clay through mechanisms such as physical adsorption, optimization of particle arrangement, and molecular chain interactions. The experimental results reveal the relationship between the improved planting and mechanical properties of the modified clay and the internal microstructural changes. This provides a reference for further exploration of new eco-friendly ecological slope protection materials.