Quantitative evaluation of defects is an important research topic in non-destructive testing. In this paper, a quantitative evaluation method for the shape of metal surface defects based on alternating current field measurement technology is proposed. The finite element numerical method is used to simulate the electric field and magnetic field environment of the test system. The rectangular probe structure is used to scan the surface of measured object according to designed strategy, and the specific shape of defects is quantitatively identified according to the obtained magnetic field signal. The results show that there are significant differences in sensitivity of eddy current magnetic fields in different directions to defect shapes, and eddy current magnetic fields in same direction are also specific for different forms of defects. The symmetry of surface defects can be determined and the cross-sectional shape of defects can be quantitatively analyzed by measuring the magnetic field scanning curve group. The surface shape of defect can be identified quantitatively by the scanning curve group after the extraction of characteristic values and proper data processing. This method provides a convenient, simple, and high precision non-contact detection method, which improves the integrity evaluation ability of metal structural parts.