Abstract: Based on the magnetic flux leakage detection technology for steel plate butt weld defect, mathematical morphology method is applied to the signal processing. A new method for the weld defect leakage detection signal processing was studied. Through the numerical signal processing, the gray graphics were obtained, and then a clear image was gotten by image sharpening. The basic characteristics of weld defect were finally obtained by image binarization for subsequent processing preparation, image thinning and the image morphology processing after the thinning.