In Ledong-Lingshui slope-break belt of Qiongdongnan Basin in western South China Sea,the stratigraphic dip changes a lot,the sedimentary facies is complex and there is abnormal high pressure zone.All these factors make the velocity analysis complicated while velocity is the key to convert time to depth and to predict pressure.So it is important to ravel out the velocity change rules and influence factors in the slope-break belt.For this purpose,the authors analyzed the seismic velocity volume and well sonic data to get the velocity distribution regularity and found that it corresponds to the ordinary distribution rules except for an abnormal low velocity zone spread out from Lingshui to Huangliu formation,which fades away from shallow layer to the depth and from downslope to upslope.In combination with the geology and pressure information,it is found that the main factors influencing the velocity change rules include depth of water,formation dip,pressure,abnormal lithologic body et al.Based on above understanding,the authors applied the slope-break belt velocity volume to three sand bodies' time-depth conversion.Different calibration cases are comparatively studied with the time-depth relationship and time/velocity/depth contour map.At last the most reasonable calibration methods were chosen for different sand proposal wells.