The developed salinity measurement setup exploits beam deviation caused by refraction due to the salinity of measured water at the receiving end face of a measurement cell. Based on a differential refractometry, a reference water cell filled with distilled water to reduce influence of temperature drift effectively is partitioned by an oblique optical-transmission plate from a horizontal rectangular water tank, which, as a measurement element, is filled with measured salt water. Beam deviation caused by salinity change is detected by a position sensitive detector. Experimental results indicate the feasibility of the developed system with a measurement resolution of 0.012‰ and repeatability error less than ±0.3‰. Errors which will affect the measurement performance are discussed and analyzed in detail. The results indicate that temperature and light power drifts have little influence on the measurement accuracy.