Holistic utilization of cotton linter black liquor is crucial from both economic and environmental standpoints. For this purpose, the hydrothermal conversion process was selected to produce hydrochar from organic materials dissolved in black liquor. Fourier transform infrared spectroscopy (FT-IR) analysis showed that there were no significant functional changes in the hydrochar compared with black liquor solid (BLS) at different preserving temperatures. However, a C-O bond was ruptured by the hydrothermal carbonization. Thermogravimetric analysis also showed that the thermal stability of the hydrochar was increased. The higher heating value (HHV) of hydrochar at the different preserving temperature from 200 °C to 280 °C was higher than BLS, reaching a maximum at 200 °C. On the other hand, the alkali from the liquor production of hydrothermal carbonization was recovered by causticization; the highest causticizing efficiency (CE) was 45.2%. The recovered liquor alkali can be used in pulping or pretreatment strategies.