<p>Fe-Si alloy is an important soft magnetic alloy material in industrial production. Fe-Si alloy must be decarburized in the production process. Decarburization can not only improve the magnetic properties of the alloy itself, but also improve the corrosion resistance and processing performance of the alloy. The essence of decarburization is the diffusion of C atoms in the Fe-Si alloy. Therefore, it is very significant to study the diffusion process of C atoms in Fe-Si alloy. In this paper, the influence of Si content on the diffusion of C atoms is studied by first-principles. The interaction between C atoms and Si, the properties of bonding electrons in Fe-Si alloy and the diffusion barrier of C atoms are obtained. At the same time, the linear relationship between the diffusion barrier of C atom and Si content was obtained when the Si content was 1.56 at.% ~ 5.47 at.%. Through this linear relationship, the relationship between the diffusion coefficient of C atom and Si content and temperature is also obtained. Finally, the diffusion constant, diffusion coefficient and diffusion activation energy of C atoms in Fe-Si alloy were obtained by expression calculation, and the influence of Si content and temperature on the diffusion process of C atoms was quantitatively analyzed.</p>