This study presents the fabrication and characterization of a plastic optical fiber-based Au/ITO film layer SPR reflection probe, enabling precise measurement of seawater salinity with enhanced sensitivity. The optical properties of the D-shaped optical fiber SPR sensor with ITO coating were investigated through parameterized scanning using numerical simulation, while the plasmonic resonance effect of the Au/ITO film was validated under 8 typical optical mode field distributions. The change in refractive index response of the numerical model was analyzed when modifying the thickness of the ITO film, particularly focusing on the impact on the surface plasmon resonance wavelength position. A multi-mode optical fiber SPR reflection probe with Au/ITO coating was prepared using thermal evaporation deposition technology, and the refractive sensitivity of the sensor was evaluated through testing in glycerol solution. Experimental results dimonstrate that within the refractive index range of 1.33–1.39 RIU, the sensitivity of the Au/ITO film SPR sensor increased to 3557.82nm/RIU, which is 57% higher compared to the Au film layer sensor. Additionally, a dedicated mechanical housing was designed to encapsulate the optical fiber sensor, and seawater salinity tests were conducted, demonstrating high sensitivity (588 pm/‰) within the salinity range of 12.2‰ to 124‰. Therefore, these combined theoretical and experimental findings suggest promising prospects for the practical application of this sensing probe in the measurement of seawater salinity.