In this work, we investigated the fractionation of raw biomass based on a “lignin-first” biorefinery strategy that enables the fractionation of corn stover into a stable lignin oil and a cellulose-rich pulp. Lignin fraction was completely disassembled into monomers with a yield of 25.1 wt% in methanol with a H₂WO₄ catalyst at 200 °C for 6 h. Higher temperature (≥220 °C) and longer reaction time (≥6 h) were in favor of higher delignification and hemicellulose removal, but led to lower cellulose retention and lignin monomer yield. Particle size of corn stover has minor impact on the delignification but significant for hemicellulose removal. A two-step mechanism is proposed for the fractionation of corn stover, in which ether and ester bonds between lignin and hemicellulose cleave to form oligomers through methanol solvolysis first, while the depolymerization of lignin oligomers to methyl hydroxycinnamates, e.g. methyl p-coumarate and methyl ferulate, as the major products happens in the catalytic steps.