Abstract: The sound radiation into semi-infinite water space from a water-filled pipe opening is theoretically analyzed. The opening is attached to an elastic baffling panel. In this analysis, the traveling wave method is used to deal with the wave propagation in water-filled pipe, and the coupling between the pipe wall and water is reduced to finding dispersion curves in water-filled pipe. The modal expansion method is applied to describe the vibration response of the attached plate, and the water loading above the plate is ascribed to calculate the radiation impedance of the corresponding modes. Numerical result shows that both the fluidic and structural modes are important to the sound radiation.