Reductive catalytic fractionation (RCF) is a promising technology that can selectively extract lignin in biomass and depolymerize it. Here, we prepared one low Ru loading catalyst (Ru_0.8/C) for RCF of biomass to selectively produce different lignin oils (both monomers and oligomers) under different reaction atmospheres. The yield of phenolic monomers reached 46.0wt.% (rich in 4-propylguaiacol and 4-propylsyringol) in the RCF of birch wood under high H₂ pressure and using methanol as the solvent. But, under N₂ atmosphere the dominant monomers shifted to 4-(prop-1-enyl)guaiacol and 4-(prop-1-enyl)syringol (79.6% selectivity) with 35.8wt.% yield of total monomers using the same catalyst. The developed catalyst can also transform native lignin in other biomasses such as pine and corn stover. Mechanistic investigation using different model compounds and deuteration indicates that removal of the C_γ─OH in methanol-extracted lignin fragments occurred before obtaining the monomeric lignin fragment, and the C_α─OH and β─O─4 bonds were then cleaved simultaneously to form 4-(prop-1-enyl) substituted monomers. The results are distinct from the reported mechanism that the precursors of the propyl and prop-1-enyl substituted monomers are monolignols with removal of the C_γ─OH at the monomeric level. Thus, this work provided novel insights into the reaction pathway of (native) lignin depolymerization.