Nondestructive testing of the surface and subsurface of welding defects is key for ensuring the quality of welding products. A three-dimensional （3D） reconstruction method of welding defects based on Faraday magneto-optical imaging （MOI） is investigated to realize the shape and size recognition of welding defects. First， based on the principle of MOI， the corresponding relationship between the magnetic induction intensities of the magnetic leakage field and MOI is analyzed. Subsequently， using a pulsed laser welding pit （3 mm × 0.3 mm × 0.25 mm） as the research object， a 3D finite element magnetic field simulation model of the pit is established to investigate the distribution of magnetic induction intensity of the leakage field. Moreover， a two-dimensional plane contour of welding pit defects is extracted via image digitization and the pixel value distribution of MOI. A gradient-deviation algorithm is designed to construct the depth information. Finally， the 3D profile of the welding defect is obtained. Results show that the magnetic field intensity should be greater the farther it is from the center point of the welding pit defects. Meanwhile， the closer it is to the center point of the Y-axis direction， the larger is the gradient of the field intensity change. The maximum depth of the pits is between 150 and 200 μm， and the differences in the average and median depths are 0.1 and 2 μm， respectively， which are different from those of a confocal microscope. MOI technology affords high identification accuracy and can realize the 3D contour reconstruction of welding defects.