When a light beam passes through the liquid, there will be a shading effect for the total reflection at the interface between the liquid and the air, which can be formed into a round shading pattern, and the radius of the shading pattern is concerned with the liquid refractive index. Based on this principle, a device of measuring seawater refractive index is established with He-Ne gas laser as the light source, and the shading pattern is detected by a digital camera with high resolution. The refractive index of seawater is calculated by MATLAB digital image processing program. Furthermore, the relation between the refractive index and the temperature is investigated, the results indicate that there is a linear relationship between seawater refractive index and change of the temperature. Research results show that the method is characterized by its easy achievement, good repeatability and high measurement precision, the application of seawater refractive index measurement is feasible.