This research is focused on the fluorescence properties and reaction conditions of Fe (Ⅲ)-KI-RB system. I₃^– is generated with Fe^{3 +} reaction with excessive KI in hydrochloric acid solution, which can react with rhodamine B to form Ion complexes to lead to quenching of inherent fluorescence of rhodamine B.The results shows that excitation wavelength（λex）is 352 nm and emission wavelength（λem）is 580 nm.The fluorescence intensity decreased linearly with the iron(Ⅲ) concentration in the range of 0.02～4.50 μg/25 mL. The detection limit is 0.006 μg/25 mL. It is applied to the determination of trace iron(Ⅲ) in peanuts with satisfactory results,which the relative standard deviation is less than 2%, and the average recovery rate of iron is between 96.7%～101% when adding standard solution to the samples.