Multi-beam sounding technology is a technology that can obtain the detailed information of underwater terrain quickly and accurately, and is widely used in the fields of ocean survey, seabed geological survey, underwater engineering construction and so on. In this paper, based on the actual scene of the survey ship, the coverage width of the survey line is divided into the left and right parts. Firstly, the parameter expressions of each section are obtained through geometric relation operation, and the mathematical model of the coverage depth of multi-beam sounding is established. Then, the paper considers the mathematical model of multi-beam sounding coverage width in a rectangular sea area to be measured, when the angle between the direction of the measured line and the normal direction of the seabed slope is β. Finally, this paper designs a set of measuring lines with the shortest measuring length in a rectangular sea area with known area, sea water depth at the center point of the sea area and the open angle of the multi-beam transducer, which can completely cover the whole sea area to be measured, and the overlap rate between adjacent strips can meet 10%~20%. With the help of MATLAB for iterative calculation, we obtain the shortest measuring length of the measuring ship. At the same time, this paper also give a set of measuring beam position distribution table.