Abstract: When the magnetic yoke method is used to detect the special structure of ferromagnetic materials, such as narrow gap, weld groove and weld root area, the magnetic pole of the yoke cannot be placed in a suitable space due to the narrow detection area. The abrupt change of the structural geometry formed by the grooves on both sides will also lead to the distortion of the magnetic field path. It is difficult to detect defects by operating according to common standards and specifications. Through theoretical analysis and experimental verification, the effective magnetization specification is determined, and the key points of operation are clarified, which provides the idea and method reference for the detection process of ferromagnetic materials with similar narrow gap structure.