Abstract: At present, the detection method for 921A steel crack can not effectively use the material characteristics to obtain the optimal probe parameters, so as to improve the detection accuracy and effectively reduce the material loss in the detection process. In this paper, a non-destructive testing system for 921A steel crack is developed and tested based on alternating current field measurement (ACFM). The model was established and simulated by the finite element software Ansys Maxwell, and the response surface methodology (RSM) was used to obtain the optimal electromagnetic combination parameters of the probe. Combined with the actual situation, the single-chip microcomputer is used to design the magnetic core excitation and power amplification circuit, the probe detection circuit, and the signal control software is written. The Matlab is used to design the detection signal acquisition and data processing software to form a complete detection system. Finally, different sizes of cracks are used for detection. The experimental results show that the output voltage of the signal generator in the system is stable, and the power amplification module can reduce the inductance enhancement excitation effect, and can effectively detect different crack characteristics. The length error rate is 7.325%, and the depth error rate is 9.96%.