In the process of operation engineering of offshore floating structure system, one or more shuttle tankers are often needed to carry out oil and gas outward transportation in parallel with it. Therefore, there will be narrow gaps which are much smaller than the structural scale, and then lead to the problem of fluid resonance in the narrow gaps. In this paper, a numerical method is used to study the fluid resonance between narrow gaps under the action of three parallel tanks with different drafts. The influence of the different positions of the large draft square box on the resonance wave height in the gap, the transmission coefficient and reflection coefficient of the three-box system, and the wave force on different boxes were considered. The numerical results show that the reflection coefficient in front of the three tank system is the largest, and the wave energy cannot enter the three tank system, which leads to the minimum resonance wave height in the gaps of the three-boxes system, and the minimum horizontal wave force and vertical wave force on the three-box system, indicating that the upstream large tank 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, and the phenomenon of double peak appears. By comparison, it is found that 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.