Abstract: By changing the content of particles below 5 mm in the mixture, the indoor direct shear tests on silty sand gravel soil are carried out. The shear characteristics of gravel soil and the variation laws between shear strength parameters and gravel content are studied as well as the influences of the content of coarse and fine particles on the skeleton effects of coarse particles. The results show that the change of the content between the coarse and fine particles of the gravel soil causes the wrapping effects of the fine particles on the coarse particles, resulting in the higher the content of the fine materials below 5 mm, the lower the minimum and maximum dry densities of the mixture. In order to fully reflect the relationship between shear stress and shear displacement of soil samples, the shear displacement of the lower shear box is 20 mm longer than that of the upper shear box, which is not suitable for the direct shear tests on gravel soil with high fine content. The allowable shear displacement should be further increased to more than 60 mm. With the increase of particle content below 5 mm, the wrapping effects of fine particles on gravel are gradually stronger. The skeleton of coarse particles is suspended in fine particles at a large distance and it is difficult to interact with each other. The skeleton effects of gravel are weakened, which leads to the corresponding decrease of shear stress under different normal stresses. On the contrary, the decrease of particle content below 5 mm will lead to the increase of gravel content, so that the wrapping effects of fine particles on gravel are weakened, and the shear stress under different normal stresses increases accordingly. The comparative change of the content between coarse and fine particles affects the shear strength of gravel soil by affecting the skeleton effects of coarse particles. The strength of gravel soil is the result of the interaction between coarse and fine particles.