The invention relates to a method for preparing aluminum sulfate by catalyzing alkylated waste sulfuric acid with lignocellulose ester. The method includes the steps that the alkylated waste sulfuric acid is added into aluminum oxide or aluminium hydroxide to react until the pH value is faintly acid to obtain a mixed solution, and the mixed solution is filtered to obtain leached residues and a reaction solution; the catalyst lignocellulose ester and an oxidizing agent are added into the reaction solution to conduct a redox reaction, organic substances in the alkylated waste sulfuric acid are completely converted into harmless micromolecule substances such as CO2, H2O and N2, cellulosic filter residues and a refined solution are obtained through solid-liquid separation, and finally the refined solution is crystallized to obtain the industrial-grade aluminum sulfate product. The selected lignocellulose ester has the catalysis function, the oxidizing agent activating function and the adsorption and decoloration functions. No other impurities are introduced in the production process, so that secondary pollution is avoided; the organic substances in the waste acid are oxidatively degraded, the mother solution is completely recycled, and product quality is not affected and reaches the industrial-grade quality standard; the production process is easy to operate, safe and reliable and adopts normal-pressure operation, and investment is saved. The lignocellulose ester catalyst is biodegradable and can serve as a raw material of biofertilizer, and no three-waste emission exists in the production process.