The optical properties of metallic materials change with the thickness of the film thickness, resulting in the change of their spectral characteristics. If the film is deposited by thermal evaporation or sputtering, the neutrality of Ni80Cr20 film is poor when the thickness is too thin or thick. The theoretical calculation results show that increasing the proportion of chromium in Ni-Cr alloy can improve the neutrality of the thin film, while the proportion of nickel needs to be increased for the thick film. As for the opposite spectral characteristics of the thin and thick films, intentional fractionation and increasing the proportion of Ni are proposed to improve the spectral neutrality, respectively. The test result shows that the improved coating process is effective.