Excessive iron content in human body can cause human tissue damage, skin pigmentation, liver and spleen dysfunction and other symptoms. In this paper, the reaction of Fe³⁺ with o-vanillin ethanol solution to form the dark blue complex was studied. O-vanillin ethanol solution has strong selectivity to Fe³⁺. In the range of Fe³⁺ concentration 0～200 μM , the ultraviolet light absorption of Fe³⁺ with different concentrations showed a good linear relationship with Fe³⁺ concentration (R=0.99299), the detection limit of o-vanillin ethanol solution on Fe³⁺ was about 1 M.