An optical sensor based on symmetrical metal cladding optical waveguide was proposed. Utilizing the ultra high order guided modes motivated by this sensor which are ultra sensitive to the imaginary part of the refractive index of the guide wave layer as probe, and combining with the specific color rendering principle of diphenylcarbazide and hexavalent chromium ion, the trace amounts of hexavalent chromium ion in water were detected. The theoretical and experimental results show that, the limit of detection is 1ug/L which is same as the international standard. This new method has a lot of advantages, such as lower specimen, shorter time, good real-time, high stabilization, lower cost and so on.