Abstract: This work studied the influence mechanism of water content and dry density on the shear strength of Guilin red clay. A series of direct shear tests to control water content and dry density were carried out and the relationship between water content and dry density and shear strength was established. The results show that under the same dry density condition, the cohesion and internal friction angles decrease with the increase of water content. When the water content is the same, the cohesion decreases with the dry density, while the internal friction angle increases. The analysis shows that with the increase of water content, the cementation with "water stability" is weakened, causing the cohesive force to decline; when the crystalline iron oxide content in the soil is higher than the cemented state, the internal friction angle increases, and vice versa. When the dry density becomes larger, the remolded soil cannot be recovered after breaking due to the cement bond, so that the effective cementation area of the soil particles is reduced more than the degree of increase, causing the cohesion to decrease. The growth of the dry density can change the microstructure model of the red clay, where the increase of closed pores of the soil leads to a decrease of cohesive force. The microstructure of the soil sample increases with the increasing dry density, resulting in an increase of the internal friction angle.