Induced by the crack stress field, hydrogen atoms tend to gather together to form hydro-gen atmosphere in an elastic continuous medium. The average hydrogen concentration of this hydrogen atmosphere is calculated in this paper. By regarding this hydrogen atmosphere at the crack tip as an elastic inclusion, the eigen-strain of the inclusion is calculated and the influence of the stress field pij to crack dislocation density is studied. Thus the total stress intensity factor of the crack tip is obtained. We infer that the mechanism of the hydrogen embritt-lement is the enlargement of the crack stress intensity factor by this hydrogen atmosphere. The physical process of the hydrogeninduced crack propagation is also discussed. The calculated propagation rate of the crack agrees quite well with the experimental result.