By using a high-vacuum resistive heating evaporation coating machine with the materials evaporated downwards, the research of downward thermal evaporation for large-aperture optics was implemented. The silver coating was prepared at a high deposition rate by resistive heating tungsten filaments without splashing. The adhesion layer and protective layer were also thermally evaporated downwards with specially designed evaporation boats with Cr or CrNx as the adhesion layer and SiOx or SiOxNy as the protective layer. To evaluate the performance of the protected-silver coating, three kinds of protected-silver coatings were prepared and researched in details. The reflectance, cross-section morphology and environmental stability of the prepared coating samples were characterized. The results show that the coating with 3nmCr/120nmAg/0.6nmCrNx/150nmSiOxNy structure has good performances on environmental stability and spectral characteristics. Especially,it has an average reflectance of 97.8% in the 400~1800 nm spectral range , and passed the tests of 24-hour humidity (50℃/98% RH) and salt folg.