In this paper, a new low frequency (50 to 300Hz) sweeping eddy current testing (ECT) method is developed to simultaneously detect defect and measure thickness for metallic plates. The proposed method has been derived through analytical solution (AS) and verified by finite-element method (FEM) and experimental data. A new signal characteristic from both AS and FEM demonstrate that the maximum value of $\Delta \mathbf{Z}_{\text {abs }}$ (difference value between $\left|\mathbf{Z}_{\text {sample }}\right|$ and $\left|\mathbf{Z}_{\text {air }}\right|$ ) and its corresponding frequency (lower than 300Hz) can be utilized to distinguish the defect depth in metallic plate and thickness of metallic plate, respectively $\left|\mathbf{Z}_{\text {sample }}\right|$ and $\left|\mathbf{Z}_{\text {air }}\right|$ denote the amplitude of the coil impedance $(\sqrt{\mathrm{R}^{2}+\mathrm{X}^{2}})$ under the tested metal plate and in the air, respectively. Moreover, the experimental verification results of aluminum plate correspond to AS and FEM. The accuracy of the estimated thickness of the aluminum plate is within 2%, and the accuracy of the estimated defect depth is within 17.4%.