Petrochemical equipment always coated with a coating composed of thermal insulation layer and metal protective layer for thermal insulation. The application of pulsed eddy current testing technology can realize the wall thickness detection without removing the coating. However, in actual testing, the material and thickness of the metal protective layer are inconsistent, which will affect the testing results and testing errors. In this paper, a pulsed eddy current experiment platform was built to analyze the impact of different types and thicknesses of metal protective layers on the pulsed eddy current testing signal under different insulation layer thicknesses. The results show that when the metal protective layers are aluminum and stainless steel plates, the increase in their thickness or insulation layer thickness will increase the dispersion of characteristic values, but will not affect the trend of characteristic curves. The thickness of the tested part can be calculated through the characteristic curves; when the metal protective layer is galvanized steel plate, the shielding effect and induced eddy current generated by its high magnetic permeability characteristics will affect the detection signal and characteristic curve. With the increase of the thickness of the galvanized steel plate, the difference of the late attenuation of the detection signal of the specimens with different thicknesses is smaller. When the thickness of the insulation layer increases to 50 mm, the detection signals are basically coincident, and the thickness of the tested piece cannot be calculated through the characteristic curve.