Numerical calculations of range-stacked synthetic seismograms at seafloor caused by low frequency sound source at different shallow sea environments were conducted in this paper with the combination of the FFP model with inverse Fourier transform. According to the calculated results of numerical examples, the time series of seismic wave at seafloor are mostly composed of interface waves and normal mode waves. The time series of seismic wave is dominated by interface wave if the source frequency is less than the minimal cut-off frequency of normal modes. More and more normal modes appear when the source frequency increases and gradually passes their cut-off frequency respectively. The normal modes gradually become the significant carriers of source energy and surface wave weakens accordingly. The more closely the sound source approaches to the seafloor, the more efficiently the seismic wave is excited, and the interface wave prevails in the synthetic seismograms simultaneously. When the source frequency is very low, the seismic wave is more strongly generated in soft seabeds than in hard seabeds, but more weakly when the source frequency is relatively higher.