Abstract: Taking the magnetic flux leakage (MFL) image of the rectangular slot defect in the different regions of the butt weld as the research object, a new edge extraction method for the weld and the defect is proposed based on the mathematical morphology. Firstly, the classical edge detection operators extract the edge for the rectangular slot defects in the weld and the rectangular slot defects in the heat affected zone, the test results verify the inaplicability of the classical operators. Then aiming at the above-mentioned rectangular slot defect in the different regions, mathematical morphology algorithm was used to, and extract the edge by the disc-shaped configuration. The outcome indicates that the edge detection algorithm based on gray-scale expansion transform can extract the MFL signal image of the weld and the defect boundaries, the both are separated effectively, and the method has a good adaptability and practicality.