The development of an economic and sustainable catalytic system was crucial for lignin-based biorefinery. Herein, we reported a low-cost Cu/CuMgAlOx catalyst with promising activity toward lignin hydrodeoxygenation (HDO) through a H-2-free method. Supercritical methanol was used as the hydrogen donor, solvent, and reactant simultaneously. Guaiacol was employed as a representative lignin model compound to reveal the HDO mechanism of lignin derivatives. HDOs of guaiacol performed at 250, 275, 300, and 350 degrees C with durations ranging from 15 to 120 min indicated a high HDO efficiency of the catalytic system. The obtained liquid products were categorized to oxygen-containing unsaturated products (OUPs), oxygen-containing saturated products (OSPs), and cycloalkanes. A kinetic model based on a simplified reaction process containing the three following conversion steps was established: guaiacol transformed to OUPs through the initial HDO, then hydrogenated to OSPs (medium HDO), and eventually turned to cycloalkanes by the deep HDO. The deep HDO was the rate-determining step, and the apparent activation energies of the three steps were all lower than those in the literature. Phenol, 1,2-cyclohexanediol, anisole, and veratrole were the major intermediates, the HDOs of which were programed for pathway verification. Remarkably, catechol (the culprit of condensation) was not produced in this system. Overall, a detailed reaction network of guaiacol HDO was established, and the veil of Cu/CuMgAlOx-catalyzed lignin-derivatives HDO in supercritical methanol was revealed. This work paved the way for the application of Cu/CuMgAlOx catalyst in lignin-derivatives upgrading.