This study is conducted to solve the problem of large measurement errors in the critical oscillation region by the surface light scattering method and explore the ways to apply this method to measure the viscosity of Newtonian fluids in the full viscosity range. To this end, we correct the instrumental broadening effect of small angles in surface light scattering and establish the frequency-domain dispersion equation of surface waves that can precisely describe the full viscosity range of Newtonian fluids considering the dissipation effect of the bulk phases of surface waves at the gas-liquid interface in the near-critical oscillation region. The discrete fast Fourier transform is adopted to transform the time-domain correlation data into the frequency-domain data. A multivariate frequency-domain-based fitting algorithm is developed to ensure the accurate determination of the viscosity and surface tension in any viscosity range and give the uncertainty of statistical significance. This study provides the absolute viscosity measurement theory, method, and system of surface light scattering for full-viscosity-range Newtonian fluids.