Abstract: Pointed to the difficulty of qualitative and quantitative testing for the welding testing of thick and great diameter pipe, a new method was put forward using the combination of a vertical and a shear wave probe. At first, the shear wave probe transmitted the ultrasonic wave to the welding root, and if there was a certain defect, it would produce the diffraction wave, and the wave would be received by the vertical probe located on the weld surface. Analyzing the depth of the greatest amplitude diffraction wave, combining the horizontal location of shear wave probe and the characteristics of diffraction wave, the qualitative and quantitative objection would realized. The qualitative judging methods for six kinds of defects such as crack, incomplete penetration, non-fusion, welding tumor, concave and wrong side, were concluded and the classification process was described. So introduced testing method has great application value for the welding testing of thick and great diameter pipe, especially for the welding testing which could be processed on single side and single surface tube.