Based on the potential flow theory of water waves, the interaction mechanism between the free- surface and internal waves generated by a moving point source in the lower layer of a two- layer fluid was studied. By virtue of the method of Green s function, the properties of the divergence field at the free surface were obtained, which plays an important role in the SAR ( Synthetic Aperture Radar) image. It is shown that the coupling interaction between the surface-wave mode and internal-wave mode must be taken into account for the cases of large density difference between two layers, the source approaching to the pynocline and the total Froude number Fr close to the critical number Fr2 . The theoretical analysis is qualitatively consistent with the experimental results presented by Ma Hui-yang.