Due to the great application value of super-large floating body in ocean engineering, there have been many studies on it in the past 20 years. In fact, a super-large floating body is composed of many small floating bodies, and there are slits between them. The hydrodynamic interaction between the small floating bodies is a complex hydrodynamic interference problem. For the hydrodynamic analysis research of super-large floating bodies composed of many small floating bodies, traditional numerical methods have encountered certain difficulties. In this paper, the purpose of this paper is to study the hydrodynamics of double tanks in the narrow gap environment of marine engineering based on the boundary finite element method. This paper uses the numerical simulation method. Based on the successful simulation of a single cage, the double cage composed of two single cage The hydrodynamic characteristics under the action of water flow are studied, which will lay a strong foundation for the development of deepwater multi-body combined cage research. Based on the basic theory of fluid mechanics, the hydrodynamic resistance of deep-water vessels is estimated. The complex and changeable model is replaced by an ideal model, and the resistance of the cage is divided into three parts: rod , ring and net, and the resistance of the rest is ignored. A simple formula is obtained by mathematical fitting and the error is estimated within the allowable range. Follow the steps for a scientific assessment of cage hydrodynamics using accepted formulas .