Abstract: Angle beam immersion focus ultrasonic C-scan was applied to test an electron beam circumferential weld with complex construction.The relationship between axial liquid path and second focus depth in the workpiece,identification of defect wave by using longitudinal and shear wave,the effects of refraction angle and focus flare angle on detection sensitivity were investigated.The results show that the measured values of second focus depth are higher than the calculated ones and the error increases with the increment of axial liquid path.Without disturbance of surface echo wave,defect wave is easily identified by using shear wave to detect lack of penetration in the root segment.It is also found that for small entrance area,the smaller focus flare angle,the higher detection sensitivity.