The invention provides a method for treating acid-washing wastewater and metallic ions in iron and steel industry, which comprises steps as follows: acid-washing wastewater naturally flows into a water collecting well and enters a preneutralization filter chamber under the elevation action of a pump, the preneutralization filter chamber is provided with 10-200mm limestone which is in a graded distribution mode, and after the acid-washing wastewater is preneutralized by the limestone, the pH value of the effluent water is controlled at 5-6, wherein the method provided by the invention greatly lowers the chemical expenses and operating cost as compared with the conventional method of directly adding an alkali solution; the acid-washing wastewater, which is subjected to initial pH value regulation, naturally flows into an oil separation regulating chamber, so that iron ions in the wastewater react with the alkali solution to form a precipitation mixture of ferric hydroxide, zinc hydroxide and the like; and the wastewater is elevated into a membrane solid-liquid separator to complete solid-liquid separation, so that the wastewater can be discharged after reaching the standard. The invention thoroughly solves the problems that the precipitate can not easily settle, the iron can not be easily removed, and the iron ions are dissolved out. The invention has the advantages of simple technique, high economy, low requirements for technical levels of workers, and high controllability, is reliable to operate, and has wide market prospects.