Abstract: Nuclear power is recognized as clean energy, but its safety has attracted more and more attention. Finite element simulation method was used to study the relationship of eddy current impedance signals and crack defect character of the steam generator tube for nuclear power plant. The numerical simulation of eddy current testing method was performed to simulate the steam generator with the differential bobbin coil in this paper. The influences of the defect configuration, testing frequency, the crack width and depth on impedance signals were studied. Through analyzing the simulation result and comparing the eddy current testing signals, we found some important relationships between eddy current impedance signals and flaw configuration, including testing frequency, the crack width and depth, respectively. The results were very important for nondestructive testing of SG tube in nuclear power plant in-service.