In this study，the detection approach was established for spectrophotometric determination of iron (Ⅲ), existing in the form of isomaltooligosaccharide-iron (Ⅲ) complex. The method was optimized by operating experiments considering the factors of the process. Several factors effected on isomaltooligosaccharide-iron (Ⅲ) complex digestion, including the volume fraction of acid, the heating temperature and time, and this approach yielded to accurate results except the kinds of acid. The digestion was positively correlated with the acid volume fraction, heating temperature and time. It was found that over 2 mL phenanthroline solution was demanded and that the complexation could maintain stability within pH 3-8 at 240 min. Good linear correlation personated in 1-5 mg/L of iron concentration, R²=0.999 9. Iron contents of isomaltooligosaccharide-iron (Ⅲ) complex were 42.04%, 40.43% and 39.45% respectively. Therefore, a simple, rapid, sensitive and selective assay for iron (III) determination was proposed successfully by the results of average recovery and precision of the experiments.