The Three Parallel Rivers area is located in the northwest of Yunnan Province and is the core area of an important ecological barrier in Southwest China. The area has serious soil erosion, but is rich in hydropower resources, so a large number of hydropower stations have been planned and built. With the operation of the power station, the long-term and repeated changes of the reservoir's water level have made it difficult for many native plants to survive, and a large-scale water level fluctuation zone has been formed around the reservoir. The erosion of the soil in the water-level fluctuating zone is intensified under the effects of rainfall and changes in water level of the reservoir. Therefore, studying the mechanism of soil consolidation by herbaceous plant roots in the reservoir water level fluctuating zone and selecting suitable plants with strong soil-fixing ability is of great significance for restoring vegetation in the water level fluctuating zone of the reservoir, controlling soil erosion, and improving the stability of the reservoir bank. In this study, four herbaceous plants including Cyperus alternifolius L., Canna indica L., Arundo donax var. versicolor, and Acorus calamus L. with a growth period of 18 months were taken as research objects. The self-made in-situ shearing instrument was used to measure the shear strength of the root-soil complexes and the rootless soil; the root tensile testing machine was used to measure the tensile strength of the roots. The Wu and Waldron model (WWM) was used to predict the increment in the shear strength of the soil by the root system and to evaluate the reliability of the model. The results showed that:① the roots of the four suitable herbaceous plants could increase the shear strength of the soil. Compared with the rootless soil, the shear strength of root-soil complexes with roots of Cyperus alternifolius L., Canna indica L., Arundo donax var. versicolor, and Acorus calamus L. were increased by 403.89%, 248.32%, 388.50% and 200.71%, respectively; ② The tensile strength of the roots of the four herbaceous plants all showed a negative power function relationship with the diameter; ③ Herbs with higher root tensile strength, higher root area ratio on the damaged surface, and more fine roots with D<1 mm, the soil-fixing effect were more significant; ④ The calculated value of the WWM model for the 4 herbaceous plant roots to increase the soil shear strength was 9.37-43.85 times of the measured value, so the model had a low prediction accuracy for the herbaceous plant roots. In conclusion, the root systems of Cyperus alternifolius L., Canna indica L., Arundo donax var. versicolor, and Acorus calamus L. all had strong soil-fixing ability, especially the root systems of Cyperus alternifolius L. and Arundo donax var. versicolor, which were the preferred species for vegetation reconstruction and soil consolidation in Huangdeng Reservoir hydro-fluctuating zone in the Three Parallel Rivers area.