The invention discloses a method for recycling waste acid. The method comprises the following steps: discharging the waste acid to a waste acid buffer tank through an acid-discharging pump and transporting the waste acid to a reaction tank through a pipeline; carrying out a reaction among waste acid containing hydrofluoric acid and nitric acid, sodium sulfate, and silica slag to obtain a solid-liquid mixture containing the nitric acid and a sodium fluosilicate solid; transporting the solid-liquid mixture to a centrifugal machine through a slurry pump to obtain liquid mixed acid and solid sodium fluosilicate; transporting the liquid mixed acid to a diammonium phosphate device through a mother liquid transferring pump for producing diammonium phosphate. The quality of a product conforms to the national ecological fertilizer standard; the solid sodium fluorosilicate is sold to other manufacturers according to the quality. The method disclosed by the invention has the advantages of less equipment input and low operating cost; hazardous wastes are transformed into available resources, so that the production cost of the diammonium phosphate is reduced; in addition, the stock amount of solid waste silicon slag is reduced and economic benefits are created.