Abstract: To evaluate the properties of the slip interface between two solids, its stiffness coefficients and the reflection coefficients of ultrasonic longitudinal and shear horizontal waves at vertical incidence are derived based on QSM model and the constitutive equations of viscous fluid, where the reflection coefficient for longitudinal wave is dependent on the adiabatic bulk module, viscous coefficient of interface and frequency of the wave, but for SH wave it is independent on the adiabatic bulk module and frequency. Accordingly, the viscosity of slip interface can be estimated directly by SH wave without any frequency limits. As to the adiabatic bulk module, additional evaluation by longitudinal wave must be employed too. The experimental results obtained from slip interfaces simulated by honey and water demonstrate the validity of the theory.