Abstract: The second-order low-frequency oscillations inside a rectangular harbor induced by different wave groups with the same resonance frequency are simulated by the full nonlinear Boussinesq model. It is found that low-frequency oscillations could be evidently excited only when the ratio of the short wavelength to the harbor entrance is greater than 3.3. The second-order wave could be as large as the first-order wave inside the harbor. The paper interprets this phenomenoncan from the interactions beteen short waves and the harbor entrance. The wavelet-based bicoherence spectrum is employed to analyze the wave-wave interactions. It is shown that the second-order long wave not only participates in wave-wave interactions between the carrier short waves but also takes part in interactions between super-harmonic wave components.