To find alloy nanoshells with better light absorption and backscattering properties, we quantitatively analyze the effects of the core radius, shell thickness, alloy composition, and ambient medium of Au-Ag alloy nanoshells on the light absorption and backscattering properties. In the process, the Mie scattering theory of double-layer concentric spheres and the size correction model of dielectric functions are adopted. The results show that when the Au molar fraction of 50%, the size step of 0.01 nm, the inner core is SiO₂ and vacuum, the maximum volume absorption coefficients of Au-Ag alloy nanoshells are respectively 93.660 μm ^-1 and 99.316 μm ^-1, with the inner core radius of 27.89 nm and 28.02 nm and the shell thicknesses of 3.95 nm and 3.35 nm. When the maximum volume backscattering coefficients are respectively 5.280 μm ^-1 and 5.550 μm ^-1, with the inner core radius of 56.08 nm and 56.37 nm and the shell thicknesses of 10.47 nm and 8.89 nm. When the Au molar fraction is less than 9%, the light absorption characteristics of Au-Ag alloy nanoshells are better than those of Au nanoshells. When the Au molar fraction is lower than 11%, the Au-Ag alloy nanoshells are superior to Au nanoshells in backscattering characteristics.