Abstract: The direction of beam width-effect has a significant influence in bathymetry.The slope angle of seabed is the key problem of the beam width-effect correction in bathymetry.Based on the theoretic analysis of beam-width effect,a slope angle calculation model was proposed,which consideres the direction of survey lines.The results of numeric simulation in different headings show that the model is valid and could get the most accurate slope angle when the survey line heading is the depth gradient direction.