Diffusion coefficient is in significant dependence on vacancy concentration due to that migration of vacancy is the dominant mechanism of atom transport or diffusion in processes, such as void formation, dislocation movement and solid phase transformation. This study aims to investigate the effect of vacancy concentration on hydrogen diffusion in alpha-Fe by molecular dynamics simulations, especially at low temperatures and with loading. Comparisons of the diffusion coefficients between alpha-Fe with a perfect structure and different-concentration vacancies, as well as comparisons between experimental and theoretical results had been made to characterize and summarize the effect of vacancy on hydrogen diffusion coefficient.