Co-hydrothermal carbonization (co-HTC) of N-rich and lignocellulosic biomass is a potential way to produce hydrochar with high yield and quality, but the nitrogen will also enrich in a solid product. In this study, a novel co-HTC with acid–alcohol assistance is proposed, and the model compounds bovine serum albumin (BSA) and lignin were used to investigate the role of the acid–alcohol-enhanced Mannich reaction in nitrogen migration. The results showed that the acid–alcohol mixture could inhibit nitrogen enrichment in solids and the order of the denitrification rate was acetic acid > oxalic acid > citric acid. Acetic acid promoted solid-N hydrolysis to NH4+while oxalic acid preferred to convert it to oil-N. More tertiary amines and phenols were generated with oxalic acid–ethanol addition and then formed quaternary-N and N-containing aromatic compounds through the Mannich reaction. In the citric acid–ethanol–water solution, NH4+and amino acids were captured to form diazoxide derivatives in oil and pyrroles in solids through both nucleophilic substitution and the Mannich reaction. The results are able to guide biomass hydrochar production with the targeted regulation of nitrogen content and species.