The structure and the magnetic properties of -Fe(Si) phase precipitated in the Finemet alloy, and some factors influencing the magnetic properties of -Fe(Si) phase are investigated by first-principles calculation based on the density functional theory. From the angle of electron spin, the magnetic properties of -Fe(Si) phases with different solubilityies of Si located at the different sites of the -Fe superlattice are investigated. The calculation results show that the spin-state density is a critical factor influencing the magnetic properties. It is found that -Fe(Si) phase with Si at the vertex sites shows better magnetic properties than those with Si at the core sites. It is concluded that with the solubility of Si in the -Fe(Si) phase increasing, the probability of Si occurring at the vertex sites increases, which results in better magnetic properties. The calculation results accord with the previous experimental results. The present work is helpful for understanding the magnetism of the Finemet alloy.