Abstract: The conductor of in-vessel coil (IVCs) in Tokamak is three-layer tubular structure. It consists of a water-cooled copper alloy as an inner tube surrounded by ceramic insulation and a stainless-steel alloy jacket tube. During manufacture of IVCs, eccentricity may occur between the copper tube and stainless-steel tube. It leads to thickness change in the insulation layer and may seriously influence the performance of the entire IVC. Therefore, it is crucial to detect and quantitatively evaluate the eccentricity via online non-destructive testing means. In this paper, an eddy current testing (ECT) based method is proposed to detect the eccentricity of IVC conductors. A finite element model for multi-frequency eddy current testing of IVC eccentricity is built based on the reduced magnetic vector potential method (Ar) to investigate the dependence of the ECT signals on the tube eccentricity. In addition, a preliminary experiment is also conducted to check the feasibility of the proposed method for practical application.