Abstract: In traditional eddy current testing, the optimal excitation frequency is difficult to determine and deviations are usually encountered during practical examination. To solve this problem, an inner coil and through-type eddy current testing model for a 1Cr18Ni9Ti stainless steel tube with 19 mm diameter and 2 mm thickness was established using the common finite element software ANSYS. According to the characteristic of differential probe, the real and imaginary part of the electric current, I_REAL and I_IMAG, and the impedance difference Z of the coil with different distance from defect were calculated and then the simulation signal of the artificial defect were obtained. After a careful comparison between the amplitude and phase of the signal of different frequencies, the optimal testing frequency of through-type eddy current was determined as 50 kHz.