Seawater is a complex system, which involves variety of organic, inorganic, dissolved and suspended substances. However, the main components in seawater are the inorganic salt, such as NaCl, MgCl2, KCI, NaHCO3, MgSO4, and so on. These elements have a big influence on laser′s transmission underwater. To further understand the optical properties of seawater, the transmittance in these salt waters with different concentrations of salt is measured. The transmission spectra of dissolved substance solution is measured by ultraviolet- visible- infrared spectrophotometer. The transmission characteristics of different salt waters are studied by using the wavelength of 450, 532, 633 nm lasers. Then a first- order exponential curve is gotten from experimental data, which shows the relationship between transmittance and the conductivity in different solute concentrations. From the figures, a conclusion that transmittance decreases with the increasing of concentration is gotten. With the same transmission distance and different concentrations, the attenuation characteristics of light are different. The work is hoped to be helpful for the research about underwater target detection and laser communication in the sea.