There is a linear corresponding relationship between the refractive index of seawater and the salinity at a certain temperature, so the salinity can be calculated by measuring the seawater’s refractive index indirectly. Several ways to measure the salinity indirectly through the refractive index measurement using optical methods were introduced particularly, including fiber Bragg grating sensor detection, surface plasma wave detection techniques and detection technologies based on geometrical optics method. The salinity sensor structures, measurement precision of every salinity measurement methods were discussed, the factors that affect measurement accuracy were analyzed, as well as the problem in practical applications. Finally, the feasibility of seawater salinity real-time detection by optical technology was prospected.