Salinity measurement with a reflex optical and electrical dual-differential sensor is experimentally developed. The sensor exploits the beam deviation due to optical refraction at the receiving endface of the measurement cell, which is caused by changes in the refractive index with different concentrations of saltwater. With the help of a right-angle reflecting prism and in the presence of a referenced distilled water cell with an inclined optical window, optical differential measurement of incident beam deviations is implemented, and the deviation caused by changes in salinity are detected by a position-sensitive detector (PSD) with an electrical differential signal processing technique. Experimental setup and measurement principles are discussed. Salinity measurements based on this method avoid the influences by light power fluctuation of the incident light and temperature drift. Experimental results indicate that the estimated salinity measurement resolution can reach 0.012%.