Abstract: Black magnetic iron mud which could be used for iron smelting was prepared by oxidation co-precipitation method from the treatment of pickling waste liquid with a mass fraction of total iron at 3.0%~15.0%.The iron content of final product was used as the evaluation index to evaluate the treatment process parameters，including neutralization temperature，oxidation dilution ratio，oxidation temperature，oxidation pH，the amount-of-substance ratio of ferric/ferrous，oxidation gas velocity， dehydration pH，dehydration dilution ratio，and dehydration temperature etc.，so as to obtain the optimal conditions.Under the neutralization temperature of 45 ℃，oxidation dilution ratio of 10.0，oxidation temperature of 45 ℃，oxidation pH of 8.0，the amount-of-substance ratio of ferric/ferrous of 1.7，gas velocity of 1.2 L/min，dehydration synthesis pH of 8.0，dilution ratio of 10.0，and dehydration temperature of 107 ℃，iron mass fraction of the as-prepared product from the pickling waste liquid reached 57.4%.The technique had the advantage of effectiveness，and will bring out great economic profits and social benefits.

Key words: pickling waste liquid, magnetic iron mud, oxidation coprecipitation