A refractive index sensor based on the trapezoidal dielectric grating and the metallic film structure is proposed. The reflectivity of liquid to be analyzed is studied by the finite element method with different grating thicknesses, trapezoidal parameters and refractive indices. The angular sensitivity of the sensor is derived by optimizing the structure parameters. In consideration of the doubling effect of the angular sensitivity caused by the symmetrical movement of reflection resonance peaks, the angular sensitivity can reach 845.23(°)/RIU as the refractive index of liquid to be analyzed is changing from 1.33 to 1.34, and 1283.14(°)/RIU as the refractive index is changing from 1.34 to 1.35. The sensor has detection applications with a wide range of refractive index. The trapezoidal parameter plays a key role in the angular sensitivity of the sensor. A standing wave structure of the electric field distribution with the largest contrast corresponds to the highest sensitivity of the sensor structure.