Abstract: The second-order long wave oscillation phenomena inside an elongated rectangular harbor induced by bichromatic wave groups are simulated with a fully nonlinear Boussinesq model. Based on a low-frequency wave separation procedure, this paper investigates how the amplitudes of bound and free long waves and their relative components change with respect to the wavelengths and amplitudes of the incident short waves under the condition of the lowest four resonant modes systematically. It shows that for the given harbor and the given ranges of the short wave frequency and amplitude, the amplitudes of bound and free long waves increase with the short wavelength; and the ratios of them in the first mode are inclined to be larger than those in the next three modes. For all the four resonant modes, both of the amplitudes of bound and free long waves change quadratically with the amplitudes of the incident short waves. However, the ratios of them are almost not affected by the short wave amplitudes.