Salinity of seawater is one of the most important ocean parameters. Salinity of seawater is mainly obtained by conductivity measurement using CTD (Conductivity-Temperature-Depth). Conversion accuracy between conductivity and salinity relies on the assumption that components of seawater are fixed, as well as high accuracy and synchronism measurements of conductivity, temperature and pressure. The study of seawater salinity based on the V-block optical refractive index method provides a total different principle for salinity measurement. Achieving high resolution seawater optical refractive index measurements could help to study factors affecting the accuracy of salinity measurement. In this paper, the various instrument parameters that affect the accuracy of seawater refractive index measurement are analyzed and the optical refractometer is optimized based on the components on the shelf. This paper systematically analyzed the resolution and tolerance of refractive index measurement on the parameters of V-block refractometer, such as incident angle, external environment and prism refractive index, etc. The optical refractometer with an air film layer on both sides of the V-block was proposed for seawater salinity measurement. With such an optimization, the measurement accuracy is further improved and the tolerance is increased. The theoretical resolution to the seawater refractive index and salinity are 1.8×10-6 and 0.01‰, respectively. Experimentally, we have achieved 3.9×10-6 and 0.021‰ respectively, and a good linearity. The difference between theoretical and experimental results are analyzed.