With the wide application of portable coating Thickness Gauges, the requirements for material protection in the chemical industry, processing industry, and manufacturing testing field are also increasing. The portable coating thickness measurement technology is also developing rapidly, which can be fast and non-destructive. Accurately detect the thickness of coating and plating coating. Due to the high requirements for the substrate material and coating material, various coating Thickness Gauges have appeared on the market, and the commonly used ones include eddy current type, ultrasonic coating Thickness Gauge, and magnetoresistive type. This paper mainly introduces the influence of the substrate material on the test results of the magnetoresistive coating Thickness Gauge. In most cases, in order to play the dual role of protection and decoration, the surface of ferromagnetic materials generally has a layer of coating or other coverings. To improve product performance and quality, accurate measurement of coating thickness is required. Although ultrasonic echo technology can be used to measure coating thickness, its resolution is too low, only tens of microns.