A seawater salinity measurement system based on prism model multi-refraction is presented in order to improve the salinity measurement precision using optics refraction. The beam angle of refraction is expanded by using prism model other than parallel model, and is enlarged by setting multiple-prism-units further. So the measurement precision is improved greatly. The measurement system is developed after design and simulation. After laboratory experiments and sea experiments with ALEC sensor, it is indicated that the developed system is effective with a measurement resolution of 0.0012‰, repeatability error less than ±0.016‰, and measurement error less than ±0.1‰ compared with ALEC sensor.