Abstract: The relationship between the effectiveness of detecting surface defects and the coating thickness was researched with the method of eddy current testing, and the placed type probe was used to inspect a defective steel plate with different coating thicknesses. Results have shown that better testing effect for surface defects on a steel plate with different categories and different depths can be realized by the eddy current test using a placed type probe if the surface coating thickness is not greater than 2 500 μm.