A novel fiber optic sensor for accurately measuring the hexavalent chromium ion concentration is proposed in this paper. The sensor uses the absorption spectral intensity to indirectly detect chromium ion concentration. The principle is that the fuchsia, which origins from the chromogenic reaction between diphenylcartazide and hexavalent chromium, gradually deepens with the concentration increase of six valence chromium ion. The color film, which is plated on the surface of optical fiber, shows amaranth with chromium (Ⅵ) complex. The experimental results show that the spectral intensity positively correlates with chromium ion concentration over the range from 0.01 mg/L to 0.1 mg/L. The detectable limit is 0.01 mg/L, the operating time is within 10 min, and the detection rate is 96%.