Abstract: The relationships between defects of stainless steel tube of condenser and signal by doublefrequency eddy current test were introduced. By expounding the theory for doublefrequency eddy current test, and comparing the differences of these defect signals, combining with the application characteristics of defects signal, the closed relationships between common defect and impedance signal was put forward. The analysis results will guide significance of eddy current testing to the same type of condenser tube, also have a very good reference value for the further development of the theory of doublefrequency eddy current testing and practical application.