In this paper, a wide range of low refractive index (RI) detection surface plasmon resonance (SPR) sensors based on D-ring-shaped side double-core photonic crystal fiber in the mid-infrared band is proposed. A metal layer is deposited on both the inner and outer surfaces of the D-ring. The full vector finite element method is used to analyze the performance of the sensor, and its sensing characteristics are studied. The results show that the sensor can achieve low and high external refractive index sensing sensitivity in the mid-infrared band. The detectable refractive index range of the analyte is 1.20~1.38, and the average wavelength sensitivity and maximum wavelength sensitivity can reach 13717 nm/RIU and 21150 nm/RIU respectively, with a resolution of 1.94×10^-5 RIU. The designed sensor plays an important role in the fields of chemical, biological, and environmental detection.