Abstract: Aiming at the influence of angular beam-width and terrain effect on bathymetry in multi-beam sonar system. A correction method of depth survey based on average slope of terrain is proposed to solve the problem of water depth data distortion caused by different terrain conditions. Firstly, a depth measurement model was established based on the principle of multi-beam system. The average slope between the measurement point and 8 points around was used to estimate the inclination angle of the terrain at that point. Different degrees of water depth correction were made by the relationship between depth measured and seabed to obtain the real depth at the survey point. During the simulation process, the average slope of the terrain is estimated by calculating the average slope in eight directions around the survey point. The method is verified with actual terrain. Results show that it is highly practical and has significant correction effects on the impact of beam angle and terrain effects in bathymetric survey.