Utilizing the millimeter-scale symmetrical metal-cladding optical waveguide, mechanism of a large-range refractometer is proposed, which is applicable to measure the refractive indexes of gas, liquid and solid. In principle, anything transparent can be measured since the detection range is not limited. The data presented, based on the experiments on air, pure water and LiNbO3 crystal, show that the measurement precision is less than 3.0×10-3.