[Objective] The strengthening effect of herbaceous plant root system on the high liquid limit soil was explored to provide the basis for reasonable estimation of the plant root system about the effect of slope protection on the high liquid limit soil.[Methods] Root-soil complexus of tall fescue couch grass root and high liquid limit soil were taken as research object, and the influences of water content and root amount on shearing strength of the complexus were explored through a series of indoor direct shear tests.[Results] The cohesion ability can be greatly improved by adding root system to soil samples, and the internal friction angle was also increased slightly. The reinforcement effect of root system on the soil increased initially with the increase of moisture content, decreased as moisture continued increasing. There existed difference for optimal root amount with regard to reinforcement under different moisture content of the complexus. When soil moisture content was 20%~25%, 30%, and 35%~45%, the optimal root amount was around 0.3%, 0.4%, and 0.5%, respectively.[Conclusion] The herbaceous plant root system can effectively suppress the water sensitivity of high liquid limit soil. There is significant influence on different root systems for different moisture content.