Spurred by the continuous development of surface plasmon resonance (SPR) technology, optical fiber sensors based on SPR have become a research hotspot. Although single-mode fibers (SMFs) are simple and easy to manufacture, the sensitivity is quite poor. On the other hand, even though photonic crystal fibers (PCFs) and anti-resonant fibers (ARFs) can achieve high-sensitivity detection and the wavelength sensitivity is tens of times that of SMFs, they are complex and difficult to produce. Herein, an SPR refractive index sensor composed of micro-nano optical fibers (MNFs) is designed to detect analytes in the refractive index range between 1.33 and 1.43. Analysis by the finite element method (FEM) reveals that the maximum wavelength sensitivity is 49,000 nm/RIU. The SPR sensor boasting a simple structure, low cost, and high wavelength sensitivity has enormous potential in applications such as chemical analysis, environmental monitoring, and other fields.