Based on technical aspects of ferromagnetic components crack damage detection methods and instruments applications, this paper establishes a the magnetic flux leakage analysis model for V-Cracks to get the theoretical expressions for any probe point of flat ferromagnetic component surface. We use MATLAB software programming to do a comprehensive simulation, and obtain the relation curve between magnetic flux leakage normal component Hy and crack opening width, the relation curve between magnetic flux leakage normal component Hy and crack depth, the relation curve between magnetic flux leakage normal component Hy and crack length are obtained, crack magnetic flux leakage distribution characters: normal component of Hy with zero-crossing, maximum value of tangential component Hx, and tangential component Hz with tip zero-crossing. According to the leakage magnetic characteristics obtained underby the theoretical analysis, the array type detection device and test platform are designed and fabricated, and then the plate-type ferromagnetic component with V-Cracks is detected. The experimental results show that: there is a good agreement between the three-dimensional magnetic flux leakage obtained from thesis theoretical analysis and the test results, to a certain degree the magnetic flux leakage model has directive significance for the prediction and analysis of crack parameters, and the test device be developed according to the magnetic flux leakage model is able to detect cracks in ferromagnetic components.