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Hydrothermal carbonization (HTC) and Fenton pretreatment (FP) are effective methods currently being used to treat lignin. However, there are few reports of combining the two techniques and applying them in the treatment of lignin. Additionally, the synergistic effect of the two techniques on the physicochemical properties of lignin has not been widely investigated. Therefore, this study innovatively combined Fenton pretreatment with hydrothermal carbonization to investigate the synergistic effect on two types of structurally different lignans, and to reveal the changes in physicochemical properties of lignin under the synergistic effect. In this work, 13C NMR, XPS, FTIR, XRD, Raman spectroscopy, and automated calorimetry were used to determine the physicochemical properties and characterize the chemical structure of lignin. The results showed that the synergistic effect of FP–HTC increased the carbon fixation rate of herbal lignin (HL) and de-alkalized lignin (DAL) by 46.07 and 59.27%, respectively, and heating value of HL and DAL by 45.53 and 46.07%, respectively. The synergistic action of FP–HTC broke the stubborn and complex chemical barrier of lignin, and effectively enhanced the degree of aromatization and demethoxylation of lignin. It is notable that the synergistic effect of FP–HTC induced the formation of more stable 4-O-5 bonds and C–C bonds in the lignin structure, resulting in a moderate enhancement of the lignin chemical structure and material properties. In this study, the synergistic effect of Fenton pretreatment and hydrothermal carbonization is utilized to convert lignin into sustainable carbon materials, and the effect of the synergistic effect on the physicochemical properties of lignin is analyzed in detail, which provides valuable references for the subsequent research on the conversion of waste lignin into carbon materials. [ABSTRACT FROM AUTHOR]