Abstract: The detection sensitivity of eddy current testing is dependent on operating frequency. In this paper, a method was presented to determine the best operating frequency of eddy current testing based on spectrum analysis, and the digital eddy current impedance inspection system was designed .In this system, the excitation signal with the frequency sweep output and single output was developed, and the probe signal was picked up and demodulated for the real and imaginary parts, The real-time impedance spectrum of each probe can be swiftly accessed with the system, so the best operating frequency can be determined according to actual needs.