A multibeam sounding system can continuously measure through a specified survey line and stitch the results to obtain a topographic map of the seabed in the area. In this paper, we propose a method based on geometric mechanism analysis and Monte Carlo simulation to design a survey line for a specific rectangular slope area. Through geometric mechanism analysis and Monte Carlo simulation, it is obtained that the total length of the survey line is shortest when the projection angle of the survey line and the normal vector of the slope surface is 90 degrees in the horizontal plane and the overlap rate of the adjacent strips of the survey line is 10 %; then, according to the constraints on overlap rate and iterative boundaries, 34 unequal spacing survey lines are designed in conjunction with the three-dimensional geometric relationship, and the spacing of the survey lines is gradually denser with the depth of the sea floor decreasing from the west to the east. The design method is of great significance for the depth measurement of seawater in the actual sea area.