Abstract: Magnetic flux leakage inspection technology focused on rectangular defects for the theoretical study of the ferromagnetic crack defect detection in present. However, crack shapes presented in practice are very complicate. Complex shape has influence on its forming leakage magnetic field. It also increases the difficulty of evaluation for the crack defect. According to the problem, the paper classifies the crack shape, and the method is based on the Maxwell equations. Crack defect detection model is established. Finally, the leakage magnetic field of different shape cracks is analyzed by finite element simulation. It is concluded from analysis that the leakage magnetic field of different shape cracks shows different change with different depth and width. The defect assessment of crack is realized through the change of leakage magnetic field. The result can provide effective reference data for crack detection in engineering practice.