Molybdenum carbide catalysts over activated carbon with different crystal phases were synthesized under controllable carburization atmospheres and temperatures for transfer hydrogenation of Kraft lignin in ethanol to aromatic monomers. Single α-MoC1-x and crystal transformation of α-MoC1-x to β-Mo2C are observed under a CH4/H2 mixture and H2 carburization atmospheres, respectively. The mixed crystal phases of α-MoC1-x and β-Mo2C with different crystallinities under a N2 carburization atmosphere exhibit higher activity with a maximum aromatic yield of 0.5163 g/g Kraft lignin when carburized at 700 °C. The dehydrogenation reaction and Kraft lignin hydrogenation are proposed to explain the synergistic effect of β-Mo2C, base, and α-MoC1-x for the base-promoted transfer hydrogenation process in ethanol, which is confirmed by in situ Fourier transform infrared analysis. This highly efficient process is suitable for the conversion of different lignin feedstock.