Abstract: This study investigated the effects of decoupled temperature and pressure on lignin during the hydrothermal process. The effect of hydrothermal treatment on the lignin structure was evaluated, and the effects of decoupling temperature and pressure on the liquid products of the lignin were assessed under decoupling conditions. The results showed that lignin was composed almost entirely of the G-type monomer of coniferyl alcohol. After hydrothermal treatment, C–O bonds such as β–O–4 ester bonds in lignin were broken. Methoxy and aliphatic structures linked to oxygen-containing structures were converted into aliphatic carbon skeletons. The liquid phase products were initially vanillin and 3-(4-hydroxy-3-methoxyphenyl)-1-propanol, which were subsequently converted to guaiacol mainly by inter-monomer conversion and cleavage of the β–O–4 bond of the terminal guaiacyl unit of lignin. The decoupled high pressure inhibited the production of lignin liquid products and decreased the selectivity of isoeugenol in the products. The results of this paper are expected to provide more fundamental knowledge and understanding for the optimization of hydrothermal conversion process conditions of lignin.