This paper investigates decomposition of nickel slag, and find the optimum conditions of investigation. The interferences are removed by aqueous ammonia. Thus,at pH8-1 1.5,the amount of iron (Ⅲ)and sulfosalicylic acid can be determined by the absorbance of its complexes at 420 nm. The relative standard deviations is 0.27,-1.23, (n = 1 1 ),and the test for recovery rates is ranging from 98.4,-101.2,.