This paper, based on the principle of eddy electromagnetic induction, specifically the eddy current method and the magnetic method, combined with finite element simulation technology, simulates and analyzes the coil impedance and magnetic induction strength of the substrate surface, and verifies the feasibility of the inspection method. According to the study, the galvanized layer and coating thickness of the substrate surface have a significant effect on the coil impedance and magnetic induction strength. Based on this, this paper designs and develops the measurement circuit, algorithm and detection equipment for the thickness of galvanized layer and coating on the surface of ferromagnetic substrate. The experimental results show that the error of this equipment is less than 1% in the measurement of coating thickness on aluminum base surface, less than 6% in the measurement of coating thickness on iron base surface, and less than 6% in the measurement of insulator cap thickness. The coating layer detector developed in this paper can measure the thickness of the galvanized layer and the coating layer simultaneously, efficiently and accurately.