Abstract: Surface plasmon resonance (SPR) sensors have important applications in biosensing due to their label-free and real-time detection capabilities. In this study, a side-polished optical fiber SPR biosensor was designed for wide-range refractive index detection, and the influence of structural parameters on sensor performance was systematically investigated using the finite element method. First, the physical model was established based on evanescent wave theory and SPR principles; subsequently, COMSOL Multiphysics software was employed to simulate the effects of core refractive index, core diameter, sensing region length, metal film material, ambient temperature, and metal film surface roughness on sensitivity, full width at half maximum (FWHM), and quality factor. The results indicate that the core refractive index determines the detectable refractive index range; for biological samples with refractive indices of 1.315-1.443 RIU, the core refractive index must exceed 1.443. When the core diameter increases from 50 μm to 200 μm, sensitivity remains approximately 8000 nm/RIU, while FWHM decreases from 179.09 nm to 130.22 nm and the quality factor increases from 44.67 RIU⁻¹ to 61.43 RIU⁻¹. Increasing the sensing region length from 5 mm to 15 mm does not influence the resonance peak position, but a length of 10 mm achieves the optimal balance between signal strength and resolution. Among gold, silver, and copper, silver yields the sharpest resonance peak, whereas gold exhibits superior chemical stability, resulting in the best overall performance. This work provides a quantitative foundation for parameter selection in fiber SPR sensors for biological detection applications.