Abstract: A Fenton solution with different H₂O₂ concentrations was used to pretreat an alkali lignin (AL), and combined with fast pyrolysis, the change in the content of phenolic compounds containing methoxy group in light bio-oil was explored, also, the influence of the Fenton solution on the structure of the alkali lignin was also studied. The results show that the peak area of phenolic compounds containing methoxy groups in light bio-oil decreases from 7.3×10⁹ with AL (untreated alkali lignin) to 5.2×10⁹ with 13-FML (pretreated alkali lignin with Fenton solution at a concentration of 13 mL/g H₂O₂), decreasing by about 29%. While the peak area of phenolic compounds containing methyl groups and ethyl groups increases from 3.9×10⁹ with AL to 7.2×10⁹ with 13-FML, increasing by about 1.85 times. At the same time, the yield of light bio-oil increases from 22.4% to 28.7%. Through FT-IR, ¹H NMR and ¹³C NMR analysis, it is found that Fenton pretreatment can destroy the condensational structural units of lignin and reduce the content of methoxy, thus providing favorable conditions for subsequent fast pyrolysis to produce bio-oil with low methoxy content.