In order to realize the refractive index detection of substances such as aerogels and sevoflurane (an important component of anesthetic agents) in mid-infrared spectrum, and expand the detection range of the refractive index, a surface plasmon resonance sensor based on a dual-core photonic crystal fiber for low refractive index detection is proposed. The photonic crystal fiber is composed of two kinds of air holes with different sizes around the central air hole. The outermost large air hole directly contacts the substance to be measured by polishing the fiber, for real-time dectection of refractive index based on metal external coating can be achieved. The theoretical model was analyzed by the full-vector finite element method. The results demonstrate that in the refractive index range of 1.12~1.37, the operation wavelengths of the sensor are in mid-infrared spectrum between 2 505 nm and 3 181 nm. The maximum sensitivity is 12 000 nm/RIU, and the resolution is 8.33×10^-6. The proposed sensor can adopt the mid-infrared band to achieve low refractive index detection, which has obtained a ultra wide detection range and a high sensitivity. It has a board application prospect in chemical detection, biomedical sensing, water environment monitoring and other fields.