Agricultural crop-based lignin was utilized to modify phenol-formaldehyde (PF) resin to prepare fast curing biobased phenolic resins by copolymerization. Prior to the reaction, four types of demethylation reagents were used at atmospheric pressure to sever the methoxy bonds of lignin to increase its reactivity. The chemical and structural changes of the lignin molecule were analyzed using Fourier transform infrared (FT-IR) spectroscopy and ¹H-NMR. The analytical results revealed that the methoxyl groups of lignin were transformed by the demethylation reagents into phenolic hydroxyl groups. FT-IR and liquid state ¹³C-NMR analysis revealed that lignin demethylated by Na₂SO₃ resulted in more phenolic-OH groups on the molecule and higher molecular reactivity. The influence of demethylation on the properties of biobased phenolic resin was also investigated. The increased quantity of hydroxyl groups resulting from lignin demethylation caused higher curing rate of the biobased phenolic resin. These results were confirmed by the improved bond strength and lower formaldehyde emissions of plywoods bonded with biobased phenolic resins.