A theoretical model for wave propagation across solid–fluid interfaces with fluid–structure interaction (FSI) was explored by conducting experiments. Although many studies have been conducted on solid–solid and fluid–fluid interfaces, the mechanism of wave propagation across solid–fluid interfaces has not been well examined. Consequently, our aim is to clarify the mechanism of wave propagation across a solid–fluid interface with the movement of the interface and develop a theoretical model to explain this phenomenon. In the experiments conducted, a free-falling steel projectile was used to impact a solid buffer placed immediately above the surface of water within a polycarbonate (PC) tube. Two different buffers (aluminum and polycarbonate) were used to examine the relation between wave propagation across the interface of the buffer and water and the interface movement. With the experimental results, we confirmed that the peak value of the interface pressure can be predicted via acoustic theory based on the assumption that projectile and buffer behave as an elastic body with local deformation by wave propagation. On the other hand, it was revealed that the average profile of the interface pressure can be predicted with the momentum conservation between the projectile and the buffer assumed to be rigid and momentum increase of fluid. The momentum transmitted to the fluid gradually increases as the wave propagates and causes a gradual decrease in the interface pressure. The amount of momentum was estimated via the wave speed in the fluid-filled tube by taking into account the coupling of the fluid and the tube.