The magnetic switch is one of the switching devices with excellent performance that can be selected for the repetitive frequency pulse power system. At present, the simulation model of the magnetic switch is a pure circuit model established based on the macroscopic characteristics of the volt-second integral, without considering the change of the magnetic core characteristics during the core saturation process, it is difficult to accurately predict the pre-pulse on the magnetic switch load, and the front error of the waveform is also larger. In this paper, the hysteresis loop and initial magnetization curve of the Fe-based nanocrystalline magnetic core under fast pulse excitation are tested and obtained. Using the key parameters of the magnetic core hysteresis loop, the J-A parameter of the magnetic core under pulse excitation is extracted, which is used to define Magnetic core properties for a magnetic switch model in multi-physics field. For the magnetic switch pulse compression circuit, the field-circuit coupling simulation model was established by using the multi-physics simulation software COMSOL, and the output waveform was simulated. Compared with the experimental results, the pre-pulse amplitude error is 2%, the peak error is 2%, and the front error is 5%, which proves the validity and accuracy of the established field-circuit coupling simulation model.