Based on the surface plasmon resonance (SPR) effect, a fiber-optic refractive index sensor based on a multimode-singlemode-multimode (MSM) structure was designed. The structure of the MSM is constructed by optical fiber fusion, and the surface of the single-mode optical fiber is coated with a titanium dioxide/silver (TiO2/Ag) composite film to form a sensing unit. The effects of single-mode fiber length and metal film thickness on sensor performance were analyzed using FDTD Solutions. The results show that the longer the single-mode fiber length, the deeper the resonance depth; when the thickness of the Ag film is 50nm, and the thickness of the TiO2 film is 20nm, the sensor can obtain the best performance. In the range of 1.33~1.41 ambient refractive index, the sensor has a sensitivity of approximately 6875nm/RIU. Experimental results show that the optical fiber refractive index sensor has a simple manufacturing process and high sensitivity.