To improve the sensitivity of optical waveguide sensor based on evanescent wave sensing, a kind of high-refractive-index film coated waveguide optical fiber sensor is proposed by simulation and verified by experiment. In this study, we propose an optical fiber-waveguide-fiber EF sensing platform, which composes of a Ta₂O₅ coated polymer waveguide sandwiched by two multimode optical fibers, to enhance the intensity of evanescent wave and increase the sensitivity of the sensor. Through simulation results, the Ta₂O₅ film thickness was optimized. We selected 70 nm, 100 nm and 150 nm thickness for the coating experiments. With the coating thickness of 100 nm, we achieved absorption spectrum limit of detection for Rh B of 1×10^-8 g/mL. The sensor has the advantages of low cost, small volume, simple manufacture and high sensitivity, which can be widely used in various fields.