Abstract: A method for identifying the detection signals of transverse, longitudinal and slant crack defects was proposed, which was aiming at alternating current field measurement (ACFM) inspection of the main structure weld of the offshore platform. According to the ACFM principle,the characteristics and formation mechanism of the B_x and B_z eigenvectors of the transverse and longitudinal crack defect detection signals were analyzed. Through the experimental verification of the longitudinal (0°) crack defect signals, transverse (90°) crack defect signals and slant (45°) crack defect signals in relation with the direction of scanning, the ways how to identify these defects were concluded.