Abstract: The distribution of the induced current and magnetic field, the influences of the factors including the frequency, probe deflection, defect size on testing results during the alternating current field measurement (ACFM) were analyzed by COMSOL Multiphysics software. During the testing, the induced current bypassed the defect, resulting in a significant current density at both ends of the defect, and the induced magnetic field generated distortion. Optimal detection occurred when the probe surface was aligned parallel to the weld surface. The length of a defect could be inferred from the distortion range of the x and z direction components of the magnetic induction intensity, while its depth could be assessed by comparing the distortion x direction component of the magnetic induction intensity. For 316L fillet welds, the maximum distortion of the x direction component of the magnetic induction intensity was observed at an excitation frequency of 1 kHz, and the best results achieved. The results indicated that simulation analyses results were largely consistent with actual experimental findings. However, during practical tests, the rough of the weld seam surface could cause certain interference to the magnetic induction intensity signals.