A process for purifying phosphoric acid includes removing volatile components from phosphoric acid rich in impurities to form a crude phosphoric acid liquid substantially free of volatile components; heating the crude phosphoric acid liquid to a temperature above 250.degree. C. in order to decompose phosphoric acid in the crude phosphoric acid liquid and generate gaseous phosphoric anhydride; and introducing the gaseous phosphoric anhydride into water or a dilute phosphoric acid aqueous solution in order to hydrate the gaseous phosphoric anhydride to form phosphoric acid.