The invention discloses a method for treating waste acid by gypsum sedimentation, arsenic oxidizing sedimentation and iron salt neutralization and co-precipitation. The method comprises the following steps of: the first section: gypsum sedimentation, namely putting lime cream into the waste acid, and removing most sulfuric acid from the waste acid through neutralization reaction to generate CaSO4sediment and gypsum; the second section: arsenic oxidizing sedimentation, namely performing solid-liquid separation after the gypsum reaction, adding polymeric iron into supernate, adding hydrogen peroxide oxidant into the supernate to oxidize Fe2+ into Fe3+ and oxidize As3+ into As6+ under the condition of low pH, and removing most arsenic from the waste acid through oxidizing sedimentation reaction so as to enrich the arsenic in the waste acid and produce arsenic slag; and the third section: iron salt neutralization and co-precipitation, namely performing solid-liquid separation after the arsenic reaction, adding polymeric iron and heavy metal ion hunting agent into the supernate, and removing residual arsenic and heavy metal ions from the waste acid to generate neutralized slag and water meeting the national discharge standard. The method has the advantages of short flow, simplicity, practicability, good pollution control effect and good gypsum byproduct, and the removal rate of arsenic is up to 99.999 percent.