Abstract: Based on the silty clay slope in the expansion project from Fangzheng to Harbin along Tongjiang-Sanya Highway, using triaxial shear tests, field monitoring and in-situ test method, the mechanism of slope instability caused by freeze-thaw and the reinforcement by vegetation are studied. Triaxial test results show that when the soil water content is less than the optimum water content, the soil cohesion increases with the increase of moisture. The soil cohesion decreases with the increase of moisture when the soil water content is larger than the optimum water content. The peak value is near the optimum water content. The internal friction angle of soil decreases with the increases of moisture. Soil cohesion decreases with the increase of times of freeze-thaw cycles. Field monitoring results indicate that during the process of soil freezing, moisture migrates to the frozen zone. The effect of absorbing water of woody plants is more obvious than that of turf, especially for Amorpha. In-situ direct shear tests show that the shear strength of the soil with woody root is larger than that of the soil without root. In the similar location of the same slope, where Amorpha fruticosa and Lespedeza are planted, the shear strength of the soil with woody root is three times that of the soil without root. Theoretical analysis shows that it can effectively reduce the slope loading so as to strengthen the safety factor of slope stability