Abstract: For ferromagnetic materials (such as low carbon steel, low alloy steel, etc) are liable to defect or fracture in the long-term and thus to cause accidents, the internal structure of the ferromagnetic materials and change tendency are studied in the paper. By the combination of the magnetostrictive effect and Faraday principle of electromagnetic induction, a stress concentration detection system for the steel plate is built. Through weak magnetic signal conversion, acquisition, processing curve fitting, test results can be accurately and intuitively shown. Steel plate stress tests show that the system can realize detection and localization of the stress concentration regional or defects of the ferromagnetic materials; and an approximately linear relationship between the change of stress and output voltage exists.