The refractive index of sea water is one of the important parameters of ocean optical characteristics, which is of great significance for seabed imaging and underwater detection. In order to facilitate the simulation of 1 km underwater laser transmission in geomagnetic environment, we designed a seawater experiment device with a length of 1 m in the laboratory, and the longitudinal magnetic field intensity in seawater test unit is from 0T to 0.05T generated by energized finite length solenoid. CCD method was used to measure the refractive index of salt solution under the applied longitudinal magnetic field. It is found that the refractive index of different salt solutions varies with the intensity of applied longitudinal magnetic field. In combination with the experiment, it is proposed that the electron cloud polarization is caused by lorentz force when the magnetic field intensity increases, and then the refractive index changes. It is preliminarily concluded that the refractive index of salt solution increases with the increase of longitudinal magnetic field strength. Origin software is used to analyze and fit the test data. The relationship formula is obtained, which provides a theoretical basis for the influence mechanism of geomagnetic environment on the optical properties of seawater.