Herbaceous vegetation plays a significant role in regulating soil hydro-mechanical properties, making it of great value in slope protection and ecological restoration. Through rainfall infiltration model experiments and direct shear tests, the effects of the root systems of two typical herbaceous vegetation species, Tall fescue (Festuca arundinacea) and Bermudagrass (Cynodon dactylon), on soil moisture, matric suction, water retention characteristics, unsaturated permeability characteristics, and shear strength were investigated. The results show that the root systems of herbaceous vegetation significantly reduce the soil’s saturated water content by absorbing water and occupying some of the soil pores, while also delaying the rapid increase in soil moisture during rainfall. During rainfall, the root systems of herbaceous vegetation slow down the decline of soil matrix suction, enhance water retention capacity, and reduce the unsaturated permeability coefficient. The cohesion of root-soil composites exhibited an initial increase followed by a decrease with rising soil moisture content. Specifically, peak cohesion values for both Tall fescue and Bermudagrass root-soil composites occurred within a critical moisture range of 16%~18%. As the moisture content increases, the influence of herbaceous vegetation roots on the soil’s internal friction angle strengthens, with the difference in internal friction angle between bare soil and root-soil gradually increasing. The two herbaceous species root systems exhibit differentiated regulatory mechanisms governing soil moisture dynamics and soil shear strength characteristics. Tall fescue roots enhances soil shear resistance in hydrated states through its deep coarse-root system, while Bermudagrass demonstrates superior cohesion enhancement at low moisture levels via shallow fine-root networks. This investigation elucidates the structural-functional linkages between herbaceous root system architecture and performance evolution in unsaturated soil matrices, providing a theoretical basis for ecological engineering design.