Abstract: This paper establish the simulation model of alternating electromagnetic field defect detection with COMSOL Multiphysics software. Based on the simulation model, the influence of different size magnetic core on the uniform distribution of induced current and on the defect detection has been simulated. The effect of three parameters related with the magnetic core, the height, the width and the length, on the distribution of the induced current and on the signal distribution of the inductive magnetic field was studied. The effect of excitation coil with different parameters on the defect detection sensitivity was obtained. The characteristic distribution of electromagnetic field signal of the defects under U-shaped excitation probe with different geometric parameters is also obtained. The result of the optimum geometric parameters of the magnetic core is acquired. It provides the evidence to the optimal design of U-shaped excitation probe of ACFM system.