The invention discloses a method for auxiliary preparation of a biomass-based functional active carbon material by using concentrated seawater, which comprises the following steps of: uniformly mixinga biomass material and concentrated seawater, drying, performing high-temperature carbonization under an oxygen-free condition to obtain the biomass-based functional active carbon material, and mixing the obtained first washing liquid with the biomass material again for recycling. The carbon material prepared by the method can be used as a super capacitor electrode, an adsorption material and thelike. According to the method, two difficult points that seawater is difficult to treat after desalination and an activator required by active carbon preparation is expensive are solved; meanwhile, the washing waste liquid can be combined with the chlor-alkali industry, the Hussein alkali preparation method and the seawater bromine extraction process to prepare other products, so that resources are saved, economic cost is controlled, pollution is reduced and the purposes of industry-to-industry metabolism and green circular economic process are achieved. The method for preparing the biomass-based functional active carbon has the advantages of simple operation, low cost, large specific surface area of the prepared carbon material, multiple pore structures, and excellent electrochemical, adsorption and catalytic performances.