In order to improve the sensitivity of low refraction chemical monitoring, a surface plasmon resonance sensor based on photonic crystal fiber with gold film in open loop was designed in this paper. The effects of open-loop radius, pore size and thickness of metal film on the sensitivity of the sensor were studied systematically by using COMSOL Multiphysics 5.6. Finally, a low refractive index sensor with a refractive index detection range of 1.26-1.31 was designed in the working band of 2 800-4 700 nm. The average sensitivity of the sensor is as high as 22 500 nm/RIU, and the highest sensitivity is 33 000 nm/RIU. It has a good application prospect in the detection of low refractive index substances such as sevoflurane, haloether and fluorine-containing organic compounds.