Abstract: By using of the normal mode method of the acoustic scattering theory, the time-frequency characteristics of forward scattering from a rigid sphere or prolate-spheroid in the oceanic waveguide and the free space situation are simulated, and the distortion of time-frequency characteristics of forward scattering from a rigid target in an oceanic waveguide is analyzed. The simulation results show that the temporal waveform and the spectrum characteristics of forward scattering from a rigid target in a waveguide are restricted by the properties of waveguide, and the inherent characteristics of the target scattering field are modulated by the waveguide dispersion. The "stripe" on the time- frequency spectrum becomes indistinct due to the coupling of dispersive normal modes. The level of the time-frequency characteristic distortion is controlled by scales of the waveguide and target, of which the resonant system consists, and target depth in the waveguide. The distortions would be weakening with the increases of sound transmission ability of bottom and waveguide depth, so does the target scattering strength.