In this paper, the site occupations of H under different mole ratios in interstitials of α -Fe and γ -Fe are studied by the first principles method based on the density functional theory. The total energy of the steady state crystal, binding energy, solution heat, density of states, charge density difference and charge population are calculated. The interaction between interstitial H and Fe lattice is analyzed. The influences of hydrogen dissolution on electronic structure of α -Fe and γ -Fe are discussed. The results show that the dissolved hydrogen leads to the lattice distortions of α -Fe and γ -Fe, and the volume expansion ratio increases with the dissolved quantity of hydrogen increasing. The energy analysis indicates that the hydrogen preferentially occupies the tetrahedral interstitial of α -Fe, while in the γ-Fe it preferentially occupies the octahedral interstitial. The analyses of density state, charge density difference and charge population reveal that the interaction between interstitial hydrogen and Fe lattice is contributed by the H 1s orbital and Fe 4s orbital, and this interaction is relatively weak, which is one of the main reasons for lower solid solubility of hydrogen in Fe lattice.