The dynamic shear characteristics of the reinforcement–soil interface affect the stability and durability of reinforced soil–rock mixture subgrades. A series of static and dynamic direct shear tests was conducted on the soil–rock mixture–geotextile interface using a large dynamic direct shear apparatus under different rock contents (0%, 25%, 50%, 75% and 100%). The effects of normal stress amplitude (10, 20, 30, 40 and 60 kPa) and normal loading frequency (0.5, 1.0 and 2.0 Hz) on the shear response of the interface were analyzed. The test results indicate that the shear strength of the upper and lower boundaries of the interface first increases and then decreases with the increase in rock content. This is positively correlated with the normal stress amplitude, while negatively correlated with the normal loading frequency. An increase in rock content amplifies the interface dilatancy effect, while increases in stress amplitude and loading frequency reduce the interface dilatancy effect. The enhancement of the interface friction effect can be attributed to increased rock content and stress amplitude. An empirical formula for the interface friction coefficient, as a function of rock content, stress amplitude, and loading frequency, has been established. This formula coincides well with the test results.