Abstract: The connection between the curvature of tube and the sizing of the defect in small diameter tube, detected by ultrasonic, was little concerned about. Based on the sound field calculation model, scattering ultrasonic field of the beam coming from the probe and reflected by the different tube curvature wall was studied, and then different regular reflector echo signals were calculated, and the comparison of the echo computed and DAC from DL-1 sample was analyzed, the regulation of connection was found, which could support the defect sizing with second-wave reflection methods.