Abstract: In order to accurately locate and measure the defects in low-alloy plate butt weld, the D-scan image obtained by the time of flight diffraction method was processed. In order to improve the horizontal resolution of the image, accurately recognize the position and size of the defects, the synthetic aperture focusing algorithm(SAFT-D, synthetic aperture focusing algorithm-D) was introduced. Based on the geometric relationship between the defects edge and the transducers, the mathematical model of SAFT-D processing on the D-scan image was build, the SAFT reconstruction of the D-scan image was achieved. The results showed that, by the SAFT-D algorithm, the resolution of the D-scan image was effectively improved, the capacity of the defect recognition was enhanced, the defect in material could be accurately located and quantified.