A numerical method is used to study the fluid resonance in narrow gaps between three parallel boxes with different drafts under the action of wave. The influence of different positions of the large draft square box on the resonance wave height in the gap, the transmission factor and reflection factor of the three-box system, and the wave force on different boxes is considered. The numerical results show that: the reflection factor in front of the three-box system is the largest, and the wave energy can not enter the three-box system, which leads to the minimum resonance wave height in the gaps of the three-box system, and the minimum horizontal wave force and vertical wave force on the three-box system, indicating that the upstream large box can form a good shielding effect on the rear; when the large draft box is in the middle position, the resonance wave height at the first narrow gap changes the most, the phenomenon of double peak appears, and the horizontal wave force of the smaller box on the side facing the wave is the largest; when the large draft box is located in the downstream, the resonance wave height at the second narrow gap is far greater than that of the other two cases, accompanied by obvious double peaks, and the vertical wave force of the small box at the side facing the wave is also the largest; when the large draft box is located in the middle or downstream position, the force on the large draft box is basically the same.