Tar is a barrier to limit the development of biomass gasification. Catalytic steam reforming experiments using α-methylnaphthalene (MNP) as a model tar compound were carried out in the two-stage reactor system (TSR). Based on response surface methodology, the effects of TSR temperatures and the molar ratio of steam to carbon (S/C) on MNP reforming performances were analyzed using the Li-modified Ni-based catalyst (NBC). The results show that the proper introduction of H2 is able to improve significantly the MNP conversion, specially at lower temperatures. Furthermore, it is more appropriate for the modified catalysts by Li and Mg to be loaded in the first reactor due to their significant promotion to hydrocracking reactions, and it is favorable to place the Ni/Al catalyst in the second reactor for H2-rich gas. Additionally, the carbon deposition resistance of the NBC modified by Li exhibits better than that of the NBC modified by Mg.