The structure of lignin is one of the key factors affecting its depolymerization into phenolic monomers. Four lignin fractions with different molecular weight were obtained by using ultrafiltration membrane technology, and the characteristics of lignin factions with different molecular weight were determined by gel permeation chromatography (GPC), potentiometric titration, FT-IR and NMR. It is found that wheat alkali lignin with low molecular weight contained more content of phenolic hydroxyl, methoxyl function groups, and beta-O-4 linkages, which was relatively prone to depolymerize into phenolic monomers with a higher yield, although the more content of carbonyl was reported to facilitate the depolymerization of lignin. With the increase of molecular weight of lignin, the depolymerization was more difficult, and the YMC decreased. Lignin fraction with low molecular weight was favorable for depolymerization into syringyl and alkyl syringyl, while the high molecular weight lignin was prone to depolymerize into guaiacyl and alkyl guaiacyl products. Boric acid was proved effective to avoid repolymerization and improve the YMC, and the dosage of boric acid was determined by the content of hydroxyl groups in lignin.