The relations among the reflection coefficient (REC), surface roughness, and interface pressure of solid-solid coupled interface is analyzed by ultrasonic experiment on rough surfaces with the dry-coupled method. A method for deducing the stiffness of dry-coupled interface is proposed, and the REC is calculated by the quasistatic stiffness spring model. The correctness of the method is verified by model comparison and experimental analysis, and the maximum relative error is 2.04 % compared with the measured value. This model can provide dry-coupled ultrasonic testing with a reliable theoretical data for pressure estimation required to achieve the ideal coupling effect under a certain roughness. The results prove the roughness of the dry-coupled interface can significantly reduce the coupling effect, and the coupling performance can be improved by increasing the pressure between the solid surfaces. Based on the stochastic rough interface model, the influence of the interface and the interface pressure can be analyzed quantitatively, which is beneficial to the effective evaluation of the tested specimen.