Abstract: In developing alternating current field measurement (ACFM), it is hard to test large number of cracks and different materials one by one. A lot of time and workload can be saved by using numerical simulation method instead of tests, and also the former can provide technical guidance for the developers. A numerical model for electromagnetic field distribution in the process of ACFM detection is established and solved by using COMSOL Multiphysics simulation analysis software. The sensitivity of crack detection by ACFM at different excitation frequencies was analyzed. An optimizing method of the excitation frequency in ACFM detection was presented.