A novel plasmonic D-shaped optical fiber sensor with a graphene-metal nanowire array structure was designed. The refractive index sensitivity was improved on the core by using the localized surface plasmon resonance (LSPR) produced by graphene and metal nanowires. The full-vector finite-element method was utilized to investigate the dispersion relationship between the plasmonic mode and the core mode of the structure. The effects of the thickness of the metal sensing layer and the diameter of the metal nanowires on the sensor's performance were also examined. Finally, the sensitivity of three types of sensors with different structures (gold film structure, graphene-gold film-graphene structure, and graphene-metal nanowire array structure) was compared. The results reveal that adopting the optimized structural parameters considerably improves the sensitivity of the developed sensor based on the graphene-metal nanowire array structure. Specifically, when the refractive index is between 1.33 and 1.40, the design structure obtains a maximum sensitivity of 7383.79 nm/RIU and an average sensitivity of 4136.00 nm/RIU. The findings of this study serve as a theoretical foundation for the development of next-generation plasmonic optical fiber sensors.