Abstract: In view of the defect detection in ferromagnetic structure, research is focused on low frequency electromagnetic technique under broadband excitation. Finite element analysis is conducted to investigate the effect of excitation bandwidth on electromagnetic field distribution in structure. The detection signals were analyzed by similarity method, and Euclidean distance was proposed for defect characterization. Low frequency electromagnetic experiments were conducted for defect detection in ferromagnetic structure. The results show that the electromagnetic field penetrability can be improved by broadband excitation, and the spatial resolution and the ability for interior defect detection by low frequency electromagnetic technique are more powerful if using the Euclidean distance of response under broadband excitation. The research provides a potential scheme for a wide range defect detection in ferromagnetic structure.