The catalytic hydrogenolysis of lignin was an important route to depolymerize lignin into aromatic monomer products and other high added value chemicals. Herein, the catalytic hydrogenolysis of poplar lignin was carefully studied in the presence of Nb-m-Ni-n/ZnO-Al2O3 catalysts in methanol solvent. The physicochemical properties of Nb-m-Ni-n/ZnO-Al2O3 catalysts were characterized by N-2 adsorption-desorption, X-ray diffraction, H-2-temperature programmed reduction, transmission electron microscopy, high-angle annular dark-field imaging-scanning transmission electron microscopy, and X-ray photon spectroscopy techniques. The structural properties of the raw lignin, oligomers, and monomers were detected by 2D heteronuclear single-quantum coherence NMR, gas chromatography-mass spectroscopy, and gel permeation chromatography. The Nb-2-Ni-1/ZnO-Al2O3 (Nb/Ni ratio was 2:1) showed the most attractive lignin depolymerization performance, with an 87.1 wt % bio-oil yield and 22.4 wt % phenolic monomer yield. 2-(2-Methoxyphenoxy)-1-(4-methoxyphenyl)ethanone was employed as model compound to deeply study the mechanism of NbOx-Ni/ZnO-Al2O3 catalyzed lignin hydrogenolysis. It was found that the introduction of the Nb species was in favor of breaking the beta-O-4 bond and avoiding the excessive hydrogenation of aromatic rings.