Abstract: The high content of nitrogen in the waste wood-based panels will result in NO_x pollution during their thermochemical conversions. This study focused on the nitrogen migration and transformation in the solid and aqueous products during hydrothermal conversion (HTC) of fiberboard. The effect of HTC temperatures and residence times on the distribution and conversion of nitrogen in the solid and aqueous products were investigated, and the conversion pathway and mechanisms were discussed. Results indicated that the exogenous nitrogen in the fiberboard could easily migrate into an aqueous phase during HTC, with only ~10% nitrogen kept in the solid phase at the HTC temperature of 140 ℃ and the residence time of 0. Further increase of HTC temperatures, some of the aqueous nitrogen migrated back to the solid phase. Nitrogen fixing ratios in the solid phase increased significantly with the rising HTC temperature until 220 ℃. This was mainly caused by the Maillard reactions for nitrogen fixation strengthened by the oxygen-rich intermediates generated from the hydrolysis of hemicellulose and cellulose. At the HTC temperature of 260 ℃, long residence times were favorable of the condensation and polymerization of intermediates hydrolyzed from lignocellulose, in which the free nitrogen in the liquid phase was also involved. Thus, the nitrogen fixation was obviously enhanced. In lower HTC temperatures, amine-N dominated the nitrogen in the solid phase. When the temperature reached 180 ℃, the nitrogen in the solid phase was converted into heterocyclic-N by the boosted condensation, polymerization, and aromatization reactions. Further increase of HTC temperatures changed the heterocyclic-N to be more thermostable. Meanwhile, amine-N remained a certain proportion in the solid products at higher temperatures, which verified that a part of nitrogen in the liquid phase was re-fixed in the solid phase. The content of aqueous nitrogen reached a maximum at 140 ℃, which corresponded to the minimum nitrogen content in the solid products at this temperature. Further increasing the temperatures, the content of aqueous nitrogen kept decreasing. Organic-N and ammonium-N were the main nitrogen species in the aqueous phase, and the changes of aqueous nitrogen were determined by deamination and Maillard reactions. At lower HTC temperatures, the increase of temperatures enhanced the deamination reaction and thus promoted the generation of ammonium-N. While at higher temperatures, the enhancement of Maillard reaction converted a part of ammonium-N to organic-N again. Testing on the organic-N showed that the structure of nitrogen in the aqueous phase at lower HTC temperatures was mainly large molecular structure with a high degree of polymerization. Increasing the temperature to 180 ℃ could trigger the massive formation of volatile heterocyclic-N, and the pyridine-N kept accounting for the dominant proportion of organic-N.