The abundant oxygen-containing functional groups in low rank coal endow it with strong hydrophilicity, and cause the pore surface to absorb a large amount of water, which makes the transportation costs increasing and energy conversion efficiency reducing. Therefore, the clean, low-carbon and efficient conversion of low rank coal has attracted more and more attentions. Hydrothermal carbonization isa green, low-carbon, efficient, and economical low-rank coal upgrading technology. It consumes low energy and can treat organic solidsubstances with high moisture content directly. As a renewable energy source, biomass can replace fossil fuels. Through hydrothermal carbonization, the quality of biomass can be improved significantly. During co-hydrothermal carbonization of low rank coal and biomass,there will be a certain synergistic effect between them, and compared to hydrothermal carbonization of low rank coal or biomass alone, cohydrothermal carbonization can result in higher yield and carbon retention ratio. Furthermore, functionalization modification of hydrochar can further enrich its pore structure, increase the functional groups on the material surface, and obtain high-performance function-al carbon materials for gas adsorption such as CO2 and SO2. In this paper, the hydrothermal carbonization characteristics of low rank coal,biomass and their mixture were reviewed, and the main influencing factors of biomass hydrothermal carbonization were analyzed. The activation and doping modification methods of hydrochar were also summarized, and the doping methods, mechanisms and application scope ofnitrogen, sulfur doping, and co-doping hydrochar were compared. Moreover, the adsorption performance of modified hydrochar for CO2and SO2 was also introduced, and the mechanism of CO2 and SO2 adsorption on the surface of porous carbon were described. At the end,the future development directions of co - hydrothermal carbonization of low rank coal and biomass were suggested, as well as for thelow-cost preparation of gas adsorbents using hydrochar.