Optical-fiber-localized surface plasmon resonance (LSPR) sensing offers the advantages of label-free detection, low sample consumption, and long-distance sensing. In this study, using the liquid-liquid interface self-assembly method, a uniform and dense gold trisoctahedral monolayer film is rapidly assembled on the surface of an optical fiber, following which an optical-fiber LSPR sensor is successfully prepared. Characterization test results show that gold trisoctahedra are regularly arranged and uniformly distributed without agglomeration, and that the surface of the optical fiber is well coated. Simulation analysis shows that compared with gold nanospheres of the same size, the multitipped structure of a gold trisoctahedron offers a more significant local electric field enhancement effect. Combined with microfluidic technology, the refractive index of sucrose solutions at varying concentrations is successfully sensed by the optical-fiber LSPR sensor, achieving a maximum sensitivity of 312 nm/RIU and demonstrating favorable detection performance.