A biomass-based porous carbon composite material and preparation thereof and an application thereof in CO2 adsorption are provided. In the biomass-based porous carbon composite material, with a pulping black liquid solid as a precursor, by arc treatment, porous carbon structures capable of physically adsorbing CO2 and basic substances capable of chemically adsorbing CO2 are obtained; with lignin in the precursor as the carbon source, and sodium hydroxide, sodium salts, and small-molecular carbohydrate degradation products in the precursor as the template and activator, porous carbon structures are obtained by arc thermal carbonization and self-activation; the basic substances are obtained by allowing sodium hydroxide and sodium salts in the precursor to undergo arc thermal decomposition. Further, the present disclosure relates to an application of the biomass-based porous carbon composite material in CO2 adsorption.