Variation of light extinction properties of monodisperse Au-Ag alloy nanospheres with particle size and wavelength are analyzed based on Mie theory. In the theoretical calculations, the dielectric function of Au-Ag alloy nanoparticles is corrected for the effect of the reduced mean free path of the free electrons in metal nanoparticles. Three fitting equations for determining particle size and concentration methods based on the extinction properties are proposed in this paper, including resonance wavelength method, dual-wavelength extinction method, and improved dual-wavelength extinction method. The results show that as long as the extinction spectrum of the particles is measured, the particle size and concentration can be retrieved by using the fitting equations. In addition, comparing the sensitivity and the particle size range of three methods are found that the resonance wavelength method is easier and faster than other methods.