Abstract: After serving for a long time, the reinforced gravel soil subgrade is affected by the intrusion of fine particles, and the interfacial mechanical properties of reinforced gravel soil are easy to deteriorate. Currently, there is a lack of research on the interface of fine-grained polluted soil. In order to study the effect of fine particle content on the shear performance of gravelly soil, a series of direct shear tests of reinforced gravelly soil with different fine particle content were carried out. Analyze the impact of four different fine particle contents (0%, 10%, 20%, 30%) and three normal stresses (40, 60, 80 kPa) on the shear characteristics of the geogrid-gravel soil interface, and the empirical formula of interfacial shear expansion coefficient was established. Results indicate that the shear stress-displacement curves of the reinforced gravelly soil direct shear tests are all of the stress softening type. With the increase in fine particle content, the peak strength and shear strength parameters show an initial increase followed by a decrease trend. The particle interlocking force of well graded gravel soil is strong, which is manifested macroscopically as a high apparent cohesion. The dilatancy curve shows relative shear contraction, relative dilatancy and relative shear contraction stages, and the occurrence of the maximum value of relative shear dilatation lags behind. The maximum dilation angle corresponds to the peak shear strength. Through the dilation coefficientmodel, it is found that the smaller the fine particle content and normal stress are, the stronger the interfacial dilation is. The research results can provide theoretical guidance for strength design and degradation protection of highway engineering.