Abstract: For a fluid medium with density stratification, the internal wave is easily observed under disturbance of waves, and the acoustic transmission loss and the characteristic parameters of the internal solitary waves are related. The mathematical relationship between the horizontal wavenumber difference of normal modes and the characteristic length of internal waves is established according to the coupling theory of normal modes. Furthermore, a method for estimating the characteristic length of internal solitary waves based on active acoustic means is proposed. The numerical simulation results reveal that the internal solitary wave has a significant impact on the forward transmission of sound waves. At a specific frequency, the transmission loss of sound waves fluctuates significantly and is related to the wavenumber difference of the coupling between internal waves and normal modes. An experiment system for studying internal waves in a tank is established, and the forward scattering data of the internal waves are collected by using a developed broadband transient plasma sound source and hydrophones. The feasibility of estimating the characteristic parameters of the internal solitary waves based on the resonance phenomenon is verified through experimental data analysis.