The depolymerization of renewable lignin for phenolic monomer, a versatile biochemical and precursor for biofuel and/or biopolymer, has attracted increasing attention. Here, an efficient base-catalyzed depolymerization process for this natural aromatic polymer is presented with the cheap industrial solid alkali MgO and biomass-derivated solvent tetrahydrofuran (THF). Results showed that more than 13.2% yield of phenolic monomers were obtained under 250°C for 15 min, because of the excellent lignin dissolution ability of THF and its promotion effect on the catalytic activity of MgO. Simultaneously, comparison characterization on the raw material, products and residual solid using elemental analysis, FT-IR, TG-DSC, Py-GC-MS and chemo-physical absorption and desorption demonstrated that the carbon deposition on catalyst is insignificant. Whereas, the fact that thermal repolymerization of oligimer on the pore and surface of catalyst resulting in the declination of its catalytic performance is responsible for the residue formation.