The optical constants of sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), calcium chloride (CaCl2) and sodium sulfate (Na2SO4) solutions at weight fractions ranging from 0 to 16.6667% are measured by the combined ellipsometry-transmission method in the spectral range 0.3-1.0 μm. In the combined method, a modified ellipsometry method is used to measure the refractive indices of liquids, and then the transmission method is conducted to obtain the extinction coefficients using the refractive indices obtained by the ellipsometry method. The experimental refractive indices of these binary solutions are utilized to obtain a mole-fraction weighted mixing rule, which relates the refractive index of multi-component mixed-salt solutions to the wavelength and concentration of each component. The effects of wavelength and concentration are taken into account by empirical expressions of the Lorentz-Lorenz formulized molar refraction. A mole-fraction weighted linear combined mixing rule is proposed to study the wavelength and concentration-dependent extinction coefficients of mixed-salt solutions. The optical constants of NaCl-KCl and NaCl-Na2SO4 ternary solutions, NaCl-KCl-CaCl2 quaternary solutions, and NaCl-KCl-MgCl2-CaCl2-Na2SO4 multi-component solutions are measured to verify the mixing rules. The results show that the optical constants calculated based on the mixing rules are in good agreement with experimental data, which demonstrate the feasibility of the proposed mixing rules. As the experimental analysis indicates, both the refractive indices and the extinction coefficients of the saline solutions increase with solute concentrations in the spectral range 0.3-1.0 μm.