Abstract: Acoustic property of gassy sediment is an important problem for seafloor prospecting. In order to study the effect of gas bubbles on the reflection coefficient at a water-gassy sediment interface, the corrected Biot elastic moduli involving gas bubble pulsations are derived based on the Biot wave equation corrected by linear bubble oscillations. Then the reflection coefficient is derived through combining the "open pore" boundary condition. Numerical analysis shows the reflection coefficient presents significant variation versus frequency due to bubble oscillations. For wave frequency near the bubble resonance frequency, bubble resonances cause high scattering and high attenuation, which induces to a high reflection coefficient and a poor acoustic penetration at water-gassy sediment interface. And numerical analysis indicates the volume concentration, the bubble size, the wave frequency and the angle of incidence are important parameters affecting the reflection coefficient at a water-gassy sediment interface.