The invention discloses a method for directly preparing activated carbon in waste acid. The method comprises the following steps: biomass raw materials or organic matter raw materials are smashed, the smashed biomass raw materials or organic matter raw materials are mixed with an acid solution, reaction is carried out under a certain temperature, and solid-liquid separation is carried out, so that a solid product and a filtrate are obtained; the solid product is repeatedly washed with water until a neutral property is achieved and sulfate ions do not exist, and suction filtration and drying are carried out, so that the activated carbon is obtained, and the filtrate is purified industrial sulfuric acid. According to the invention, high temperature is not required, the obtained activated carbon is dust-free activated carbon, and the obtained flexible and elastic activated carbon with high water absorbability can be widely applied to the industries of purification, decolorization, adsorption, filtration aiding, carriers, a conditioner and the like. The method is extremely wide in raw material applicability, can be used for purifying waste sulfuric acid, and can purify industrial waste acid into industrial-grade sulfuric acid.