A refractive index detection sensor for four-pole suspended core fibers with a small core diameter is designed, and three different polishing structures are simulated for this fiber. The open air holes are coated with gold film to stimulate the surface plasmon resonance effect. The finite element method (FEM) is used to analyze the surface plasmon sensing characteristics of the open suspended core fiber, and the influences of the geometric parameters of the sensing metal layer and the thickness of the suspended poles on the sensing effect are studied. The single-hole polishing structure and the opposite two-hole polishing structure have a refractive index detection range of 1.31—1.42, the maximum sensitivities are 15000 nm/RIU (refractive index unit) and 16000 nm/RIU, and the resolutions are 6.7×10 ^-6 RIU and 6.25×10 ^-6 RIU, respectively. The refractive index detection range of the adjacent two-hole polishing structure is 1.31—1.40, the maximum sensitivity can reach 20000 nm/RIU, and the resolution can reach 5.0×10 ^-6 RIU. This sensor has good performance in measuring high refractive index substances, is easy to prepare, and has broad market application prospects in the future.