The invention discloses a preparation method of p-aminobenzene sulfonic acid. The preparation method includes the steps of firstly, performing reduction sulfonation, to be more specific, sequentiallyadding water, a non-proton polar organic solvent, nitrobenzene and a reduction sulfonation agent into a reactor, and heating to 95-130 DEG C to perform reflux reaction; secondly, removing salt, to bemore specific, cooling the material obtained in the first step to 0-20 DEG C to separate out inorganic salt, performing suction filtration or filter pressing, and collecting filtrate 1; thirdly, performing acidification, to be more specific, heating the filtrate 1 obtained in the second step to 30-70 DEG C, adding sulfuric acid to regulate pH to 1-3, performing suction filtration or filter pressing, collecting filtrate 2 and filter cake, and drying the filter cake to obtain the p-aminobenzene sulfonic acid. The method has the advantages that one-step nitrobenzene reduction sulfonation is achieved, the obtained p-aminobenzene sulfonic acid is high in purity, three-waste problems are avoided, and clean production is achieved truly.