Abstract: The segregation energy distributions of hydrogen in four α-Fe symmetric tilt grain boundaries (GBs) are analyzed by using molecular statics (MS), and then the shear responses of four GBs embedded with different number of hydrogen atoms at the room temperature are investigated by using molecular dynamics (MD) methods. To facilitate our quantitative analysis, the hydrogen density ρ is defined as the ratio between the number of hydrogen atoms and the GB area. At different hydrogen densities, the variations of initial critical stress of GB plasticity and GB migration displacements are analyzed, and the micro-deformation mechanisms in each GB with the presence of hydrogen atoms are analyzed as well. It is found that the hydrogen segregation energies are generally lower in GB than those inside grain, which lead four GBs to absorb hydrogen atoms in the vicinity of GBs. With the increase of hydrogen density ρ the critical stress of incipient plasticity as well as the flow stress could be reduced. Moreover, the micro-deformation mechanisms of fours GBs with hydrogen atoms are different from those of GBs without hydrogen atoms. In particular, presence of hydrogen atoms remarkably affects GB migration velocity. Thus, GB with hydrogen atoms may undergo a pure sliding deformation instead of the shear-coupling deformation for GB without hydrogen atoms. Meanwhile, in contrast to GBs without hydrogen atoms, the micro-structures of GB with hydrogen atoms drastically evolve upon loading. In addition, the diffusion and agglomeration of hydrogen atoms may lead to the formation of nanovoid in GBs.