Abstract: On Ultrasonic testing,the wedge block and workpiece surface should be well coupled.When the workpiece surface is flat,just plane wedge can be used to meet the workpiece surface coupling requirement,and for the workpiece such as pipeline and spherical tank,the wedge surface and the workpiece surface should meet the corresponding coupling condition.This paper analyzes the wedge-workpiece surface coupling requirement in GB 11345-1989 and GB 11345-2013,the similarities and differences.Geometric analysis was made for different contact types between wedges and curvature workpiece surface,and the precise formula for calculating the gaps between plane / curvature surface wedge block and the workpiece is derived,thus providing a reference for the selection of suitable wedges.