A D-shaped single-core photonic crystal fiber optic sensor based on metal surface plasmon resonance (SPR) for measuring the low refractive index range is proposed. Detection of changes in the refractive index of the surrounding medium is achieved by plating silver (Ag) in an arc-shaped area. TiO 2 is plated between the optical fiber surface and the silver layer to act as an adhesive. The sensor has a detection range of 1.19-1.31 and a maximum sensitivity of 11,000 nm/RIU. Therefore, the sensor can be used for organic chemical sensing, biological sensing, pharmaceutical sensing and other sensing applications where a wide range of spirit refractive index is required.