Based on the difference of wave impedance between underwater sand strata and rock media, a stratigraphic model of fluctuant strata containing water, fine sand, gravel sand, and conglomerate was established using Tesseral2D software for seismic numerical simulation. Then, the forward calculation with the model was conducted under different channel spacings, least offsets, wavelet frequencies, and wave velocities in rocks. Under normal circumstances with a source frequency of 400 Hz, a least offset of 5 m, and a channel spacing of 1 m or 2 m, the seismic response is significant, with constant phases and less interference waves, which also well reflects the underwater stratum interface. In combination with the geological data from the bridge site area of the Second Nanjing Yangtze River Bridge, a stratigraphic model for underwater sand strata was established, containing water, silty clay, silty sand, gravel pebble, and conglomerate. Forward simulation using Tesseral2D software shows that, with a source frequency of 400 Hz, a channel spacing of 2 m, and a least offset of 5 m, the seismic response can well reflect the underwater interfaces between different rock strata, and determine the thickness and distribution range of underwater sand strata, providing an important theoretical basis for the exploration of underwater sand strata.