Bean curd wastewater (BCW) is the main waste in the processing of bean products. It is expected to save water resources and reutilize the organic matter in BCW by using BCW instead of pure water as reaction solvent of hydrothermal carbonization (HTC). However, the mechanism of the effect of BCW on the HTC remained unknown. To investigate the effect of different BCW dosages on the HTC of biomass, the HTC experiments of reed stalk under BCW and pure water were carried out at 250℃ for 240min at liquidsolid ratio of 5~30. The elemental composition and fuel characteristics of hydrochar were analyzed in depth. The results showed that with the increase of liquid-solid ratio, the yield of hydrochar obtained under the BCW was increased from 49.27% to 63.53% while that obtained under the pure water was decreased from 46.30% to 38.30%. The C and N content of hydrochar obtained under the BCW was 68.35%~70.30% and 1.48%~3.26% respectively, which was higher than the corresponding parameters of hydrochar obtained under pure water. Moreover, the C and N recovery efficiency of hydrochar obtained under the BCW was increased as the liquid-solid ratio was enhanced. The BCW promoted the decarboxylation reaction during hydrothermal carbonization of reed stalk. Both H/C and O/C atomic ratios of hydrochar obtained under the BCW were close to those of lignite. The higher heating value (HHV) of hydrochar was up to 29.53MJ/kg. The energy densification was about 1.80. And the energetic recovery efficiency of hydrochar was increased from 0.85 to 1.17, while the fuel ratio of hydrochar was decreased from 1.09 to 0.78. When the liquid-solid ratio was less than 10, the combustion characteristics of hydrochar under the BCW were similar to those of hydrochar obtained under pure water, indicating good combustion performance for hydrochar. Thus, the liquid-solid ratio of BCW and biomass should be less than 10 when hydrochar was used as fuel.