Violent fluid oscillations occur inside narrow gaps between closely side-by-side structures, threating the safety of the operations. In the current study, fluid resonance in the gap between two side-by-side different boxes is investigated by a two-dimensional viscous-flow numerical wave tank, which can simultaneously predict the viscous damping and free-surface nonlinearity. The accuracy of the established numerical model is fore validated by the published experimental data. The upstream box keeps fixed and has six varied breadths, the downstream box heaves freely under wave actions and remains a constant breadth. The current work aims to study the effects of the breadth ratio and heave motion of the lee-side box on the hydrodynamics of gap resonance. The wave amplification in the gap, the heave motions of the lee-side box, the reflection, transmission, and energy loss coefficients, and the response time and damping time are concerned. Results show that the gap resonance frequency is insensitive to the variation of the breadth ratio. As the breadth ratio increases from 0.4 to 1.4. The maximum wave height amplification in the gap reduces more than 25%. The maximum response time of the fluid resonance occurs when two boxes have equal breadth.