Abstract: Nitric acid demonstrates the ability to rapidly depolymerize lignin, thereby facilitating the value-added utilization of papermaking black liquor via hydrothermal liquefaction （HTL）. Using alkaline lignin （AL） as the raw material, the characteristics of the lignin hydrothermal process under different reaction times, temperatures and initial pressures in the presence of nitric acid were investigated. Additionally, the migration process of carbon elements and the reaction mechanism of hydrothermal process were explored. The study indicates that the nitric acid-assisted AL hydrothermal reaction process consists of debranching reactions （10%−15%）, acid-induced precipitation aggregation （65%−70%）, and depolymerization （15%−20%）. Hydrothermal products are predominantly influenced by temperature and initial pressure, exhibiting relatively low sensitivity to reaction time. An increase in hydrothermal temperature can promote debranching reactions of hydrothermal carbon and inhabit the acid-induced precipitation aggregation of AL to form hydrothermal carbon, reducing the proportion of carbon in solid products from 77.0% to 51.6%, and increasing the proportion of carbon in gaseous products from 0.6% to 14.2%. Elevating the initial reaction pressure can inhaibit depolymerization and promote the acid-induced precipitation aggregation of AL to form hydrothermal carbon, leading to a decrease in the proportion of carbon in liquid products from 17.6% to 4.1%, whereas the proportion of carbon in solid products increases from 50.8% to 64.9%. The primary gaseous products of lignin hydrothermal decomposition are CO₂ and a minor amount of CO, which originate from the cleavage of carbonyl, carboxyl, and ether bonds during the hydrothermal process. Liquid products primarily consist of monomers, dimers, and phenolic oligomers derived from the depolymerization of AL, resulting from partial depolymerization catalyzed by acid and subsequent repolymerization of the depolymerization products. Hydrothermal carbon is mainly composed of hydrophobic amorphous intermediates formed through the softening and adhesion of AL in an acidic environment, which adsorb partially depolymerized lignin fragments and undergo cross-linking carbonization. According to the carbon distribution among the three-phase products, the optimal operational parameters are determined as a reaction time of 4 hours, a reaction temperature of 180  °C and an initial pressure of 0 MPa. Under these conditions, the solid products accounts for 50.8% of the total carbon content and the liquid products contributes 17.6%.