Abstract: Hydrothermal treatment of biomass is effective in producing hydrochar, but the product usually has a low surface area and is not suitable for direct use as an adsorbent for CO₂ capture. We report the use of chitosan as a precursor for carbon prepared by a combination of hydrothermal treatment and mild KOH activation. The effect of an additive salt (eutectic salt of KCl/LiCl with a mass ratio of 5.5/4.5) in the hydrothermal treatment and activation temperature on the porosities and surface chemical states of the obtained carbons and their CO₂ capture were studied by N₂ adsorption, XPS, SEM and XRD. Results indicated that the porosities of the carbons were increased by increasing the activation temperature. The salt additive introduced mesopores in the hydrochar and slightly reduced the surface area of the porous carbon after activation, but was useful in increasing the number of N-species during hydrothermal treatment and activation. The carbons produced using the salt additive had much larger CO₂ uptakes under ambient conditions than those prepared without the salt, suggesting that porosity is not the only factor that determines the CO₂ uptake. The CO₂ uptake on the carbon activated by KOH at 600 °C produced from the salt-assisted hydrochar was the highest (as high as 4.41 mmol/g) although its surface area was only 1 249 m²/g, indicating that CO₂ uptake was determined by both the microporosity and the active N-species in the carbon.