A D-shaped photonic crystal fiber (PCF) low refractive index (RI) sensor based on surface plasmon resonance (SPR) is proposed, and its properties are investigated by the finite element method. Gold is selected as the plasmonic material and coated on the open-ring structure to excite the plasmonic modes. The simulation results indicate that the sensor can detect the RI in the range of 1.00 to 1.29, which is a fairly wide detection range in the detection field of SPR-based PCF low RI sensors. When the analyte of the RI varies from 1.28 to 1.29, the maximum sensitivity of 10,000 nm / RIU and the corresponding resolution of 1 × 10 ^{− 5} RIU are obtained in the wavelength range of 1.6 to 2.4 μm. When the RI of the analyte changes from 1.00 to 1.29, the relationship curve between the RI and the resonance wavelength fits well, and the adjusted R² value is 0.99609. Furthermore, the sensor achieves the figure of merit of 56.98 RIU ^{− 1} and an amplitude sensitivity of 82.76 RIU ^{− 1}, respectively. Therefore, the designed sensor can be effectively applied to low RI detection, having a certain commercial potential in medical detection, organic chemistry detections, and related applications.