To screen herbaceous plants that are adaptable to and conducive to stabilizing ecologically fragile garden slopes, this study selected 9 commonly used herbaceous slope-protection plants from Baiyun Mountain Scenic Area, a renowned mountain scenic spot in South China, and conducted tests on the shear strength, anti-scourability, and anti-erosion resistance of their root-soil composites. The results showed that all the tested plants could effectively improve slope stability, and the ranking of their comprehensive soil-fixing efficiency was as follows: Axonopus compressus > Arrhenatherum elatius 'Variegatum' > Calathea insignis>Wedelia trilobata > Hymenocallis littoralis >Hemerocallis fulva > Nephrolepis cordifolia > Ruellia simplex > Pilea notata. In terms of specific performance indicators reflecting different aspects of soil stability, Arrhenatherum elatius 'Variegatum', Hymenocallis littoralis, and Nephrolepis cordifolia had the highest shear strength in the upper soil layer; Arrhenatherum elatius 'Variegatum', Hymenocallis littoralis, and Calathea insignis exhibited the best performance in the shear strength of the lower soil layer; in terms of anti-scourability, Axonopus compressus, Wedelia trilobata, and Hemerocallis fulva showed the optimal performance; meanwhile, Calathea insignis, Ruellia simplex, and Hemerocallis fulva demonstrated the strongest anti-erosion resistance. The comprehensive evaluation based on all test indicators indicated that Axonopus compressus, Arrhenatherum elatius 'Variegatum', Calathea insignis, Wedelia trilobata, and Hymenocallis littoralis are highly suitable as excellent slope-protection plants for garden slopes in Baiyun Mountain Scenic Area and South China. To accurately quantify the relative contribution rate of each evaluation index to the overall soil-fixing effect, the entropy weight method was adopted to measure the contribution degree of each index. The results revealed that the contribution rates followed the order: anti-scourability (33.44%) > upper-layer shear strength (26.03%) > lower-layer shear strength (21.31%) > anti-erosion resistance (19.22%), indicating that the importance of different indicators varied. The upper and lower-layer shear strengths are the core indicators reflecting the mechanical effects of root-soil composites, with the contribution rate of upper-layer shear strength being higher than that of the lower layer, while anti-scourability and anti-erosion resistance are the key indicators characterizing hydrological effects. Both mechanical and hydrological effects play significant and indispensable roles in the soil-fixing function of slope-protection plants. It is worth noting that the cumulative contribution rate of hydrological effect indicators reached 52.66%, which was equivalent to that of mechanical effect indicators (47.34%). Correlation analysis further revealed that there was a highly significant positive correlation (P<0.01) among upper-layer shear strength, lower-layer shear strength, and anti-erosion resistance, while anti-scourability was only highly significantly correlated with anti-erosion resistance (P<0.01). These findings collectively indicate that there is a clear synergistic mechanism between mechanical and hydrological effects during the soil-fixing process of slope-protection plants, and provide important theoretical support for the scientific selection of slope-protection plants in South China.