A novel device measuring seawater salinity is developed, based on techniques of surface plasmon resonance (SPR) and relation between its salinity and refractive index. In the device, a prism made of K9 glass is used as substrate, and a film of Ag (55nm) is deposited on one side of the prism; after that, two flowing cells are built on the Ag film, one serves as sample solution, and the other as distilled water. Under the excitation of white light, SPR occurs in two areas at different wavelengths. Although the refractive index of seawater is determined not only by its salinity, but also by its temperature, and sample solution and distilled water have the same temperature. If the difference of resonance wavelengths (Δλ) is utilized to indicate the salinity, the affection by temperature drift could be eliminated. Theoretical analysis and experimental result verify the feasibility of the device.