The present invention discloses a method for solidifying and stabilizing waste acid including steps of condensing waste acid containing phosphoric acid to reduce the volume; mixing the condensed waste acid with waste acid containing fluoroboric acid to solidify and stabilize the mixed waste acid. The pH of the mixed acid is adjusted by adding barium hydroxide as a neutralizer. The efficiency of solidifying waste acid can be improved by partially granulating and by adding solidifying agent indirectly. The method of the present invention can prevent intensely exothermic reaction caused by adding solidifying agents. Furthermore, the method of the present invention is controlled in a temperature of 30 to 45° C. to improve the polymerization of the mixed waste acid so that the efficiency of solidification thereof can be also improved.