The time-varying magnetic field characteristics during the demagnetization process of complex ferromagnetic cavity structures were studied based on computational electromagnetic simulation. By establishing a simulation model of the complex ferromagnetic cavity structures and the magnetic field generation coil, the main factors affecting the time-varying magnetic field characteristics were analyzed and explained, including eddy current effects, hysteresis effects, material properties of the complex ferromagnetic cavity structure, and structural gap connections. The magnetic field amplitude at typical locations was investigated, and the temporal variation of the internal magnetic field was analyzed. Additionally, the evolution motion of eddy currents during the dynamic demagnetization process was simulated. It was found that the aforementioned factors significantly affect the internal magnetic field of the complex ferromagnetic cavity structures during the demagnetization process, and their influences intertwine, resulting in complex time-varying characteristics. Through theoretical analysis and numerical simulation, the mechanisms and rules of these influences were revealed. The research findings provide important references for optimizing the demagnetization process, improving demagnetization effectiveness, and developing equipment-level magnetic field protection criteria and design.