Abstract: The TL tandem test for detecting the defects of incomplete fusion in the big thickness narrow gap straight edge groove weld was introduced.It is easy to fail to detect the incomplete fusion defect with single probe and it is not convenient to use fore-and-aft tandem angle probe method to detect the same defects,therefore,the method of the transverse wave and longitudinal wave waveform conversion to detect the defects of incomplete fusion in the big thickness narrow gap straight edge groove weld is put forward.Results show the TL tandem method can effectively detect area-type defects,such as cracks and incomplete fusions in the weld.