Abstract: Si-rich films were deposited on pure iron and low-Si steel substrates by direct current magnetron sputtering, and then were subjected to vacuum annealing. The distribution characteristics of Si across Fe and low-Si steel substrates were studied by energy spectrum analysis (EDS) and X-ray diffraction (XRD). DICTRA software was used to simulate the diffusion models. It is found that the diffusion behavior of Si in the Fe substrate is from γ-Fe(Si) phase to α-Fe(Si) phase and the rate of diffusion is controlled by phase-boundary migration. When the content gradient of Si along the cross section is not sufficient to drive the phase interface to positively migrate, the phase interface moves back with the increase of diffusion time, and the diffusion process tends to be a uniform diffusion process with the time passing. But in the low-Si steel substrate, the diffusion behavior of Si accords with the Fick's second law of diffusion.