A method for calculating the depth of Multibeam Echosounder System (MBES). used for underwater target search is proposed based on factors such as the types and detection modes of MBES. The equal-angle and equal-spacing modes of MBES are discussed separately, taking into account the influence of beam overlap angles. Two formulas for calculating the depth of each mode are derived from the formula for transverse resolution of MBES, and the depths of different beam angles are calculated. After comparing the results, it is concluded that the detection capability of the equal-angle mode for searching underwater targets is superior to that of the equal-spacing mode. Finally, a case study is conducted to show that the detection capabilities of MBES are related to target size, water depth, and beam opening angle.