Abstract: In the eddy current testing for steel tubes, the problem that the phases of defect signals could not be distinguished existed when the magnetization intensity of the tubes was oversaturated. Theoretical analysis and experimental results showed that there were two testing mechanisms in the eddy current testing for steel tubes: eddy current effect and magnetic flux leakage effect. When the magnetization intensity of the tubes was oversaturated, the magnetic flux leakage effect was stronger than eddy current effect and was dominant, which made the phases of defect signals could not be distinguished.