Abstract: The Magnetic leakage-eddy current composite technique used in pipeline inner inspection is to integrate the magnetic flux leakage sensor and the eddy current sensor into the same probe shell. Detection and quantification of defects were realized by Hall sensor and determination of inside and outside wall defects were whereas realized by eddy current sensors. The influence factors of the impedance change of the eddy current coil include the shape of the defect itself, the magnetic properties of the material in the defect and the frequency of excitation coil. In this paper, the variation rule of inductance, resistance, impedance and impedance angle of coil under different excitation frequency is simulated by finite element method and experimental method when eddy current coils pass through defects located inside and outside the tubes. The results show that when the excitation frequency of the coil is about 200 kHz,the value of the resistance and the impedance angle of the coil is significantly different for the inside and outside defects under study, while the change of inductance value and impedance value is not obvious. The research results have laid a foundation for the further design of eddy current testing circuit.