Abstract: The nonlinear oscillation phenomena inside a narrow long rectangular harbor induced by different wave groups with the same resonance frequency are simulated by a fully nonlinear Boussinesq model. The least square method is used to decompose the low-frequency components inside the harbor into bound and free long waves. For comparison, the condition that harbor oscillations can not be induced by different wave groups with the same wave number of bound long waves is also considered. It is found that the amplitudes of bound and free long waves and the ratio of them are closely related to the short wavelength no matter whether the harbor is resonant or not. For the given harbor and primary wave frequency ranges studied in this paper，when the harbor is on the lowest oscillation mode, the amplitude of bound long waves is always lower than that of free long waves. However, the former tends to be larger than half of the latter when the short wavelengths are larger than 0.66 times of the harbor length. When the harbor is on non-resonance condition and the short wavelengths are larger than 0.56 times of the harbor length, bound long wave components tend to be larger than free long wave components.