A four-test-winding eddy current sensor for seam tracking was introduced to improve the accuracy of automatic seam tracking system for sheet butt joint. Both horizontal and vertical deviation information can be gained from testing the eddy current induced by the excitation winding. In addition to advantages such as non-touch with work pieces, high sensitivity and rapid response , the sensor is also suitable for non-magnetic material. An automatic seam tracking system based on the eddy current sensor has been developed. The principle ,processing circuit and experiment results of the system were presented in detail.