For detection of rough surface work piece by ultrasonic wave with solid coupling, the relationship between reflection coefficient, surface roughness and interface pressure of solid coupling interface is analyzed. A method for deducing the stiffness of dry coupling interface is put forward. The reflection coefficient is calculated by the quasi-static stiffness spring model. The correctness of the theory is verified by model comparison and experimental analysis. Compared with the experimental value, the maximum relative error is 3.56%. This model can provide a reliable theoretical data for the estimated value of the given pressure, which is required for achieving the desired effect in dry coupling ultrasonic testing under a certain roughness surface. Meanwhile, it is proved that the roughness of solid coupling interface can significantly reduce the coupling effect，and coupling performance can be improved by increasing the pressure between solid interfaces. By establishing a random rough interface model, the influence of the interface and interface pressure can be quantified.