Hydrogen-induced cracking (HIC) is a key problem restricting the application of ultra-high strength steel. It is necessary to analyze the distribution of diffusible hydrogen to reveal the mechanism of HIC. The site occupation tendency of H in interstitial and vacancy positions are investigated by the ab-initio method, and the stable configuration and steady state energy are obtained. The solution tendency of H atom in interstitial and vacancy positions is analyzed based on the aforementioned results. Specifically, the Mulliken population, density of states, charge density difference are calculated and used to analyze the interaction between α-Fe metal and H atom. The results show that the dissolved H tends to occupy the interstitial sites of the body-centered cubic, the weak hybridization interaction between the interstitial hydrogen and its nearest neighbour Fe atom is contributed by the H 1s orbital and Fe 4s orbital. Vacancies can capture H atoms easily and H atoms tend to occupy the isoelectric surface near the inwall of the vacancies. A vacancy defect can hold up to three H atoms which are difficult to combine with each other to form H₂ molecule by covalent bond. H atoms in vacancies and at interstitial positions change the charge distribution of the Fe lattice, which weakens the binding force of the atoms and forms anti-bonding orbital in local area. The proposed thermodynamical model allows the determining of the equilibrium vacancy and the dissolved H concentration for a given temperature and H chemical potential in the reservoir, and the calculated results are in good agreement with the actual results.