Because of the special engineering characteristics of the gravel soil, it causes more and more attention. However, there are less research about the mechanical properties of the crushed stone soil with large particle size, silty clay and slightly dense state. According to the characteristics of this type of gravel soil and the results of statistical survey, two typical gradation of slightly dense gravel soil are determined. The shear strength of gravel soil specimen under different moisture content is measured by large direct shear apparatus, and the relationship diagram of the shear force and the horizontal displacement under different water cut rate is drawn. The shear strength indexes c,φ value are determined by univariate regression and the impact of moisture content and gradation on the shear strength is analyzed. The results show that with the increase of water content, the cohesive strength of the gravel soil is decreasing, but the change trend is significant and related to the content of cohesive soil. The change of water content has little influence on the friction angle of the gravel soil. Using the specification method to calculate the bearing capacity of foundation, and comparing with the experience value, the results show that the influence on the moisture content of foundation bearing capacity, and is associated with the silty clay content of gravel soil filling.