From October 2010 to December 2012, the systematic study of the protected aluminum coating which is thermally evaporated downwards for large primary mirror had been developed. By adjusting the structure, it makes the physical nonuniformity of the bare aluminum films in the diameter of 1.8m drop to 4.18%, and the average reflectivity meets 95.09% in the band 400nm-2500nm. The optimized parameters suggest that the SiO<sub>x</sub> (1<x<2) protective film with the oxygen flow of 50sccm has excellent environmental stability, and the average reflectivity of the protected aluminum coating reduces just less than 5%. Further, the SiO<sub>x</sub>N<sub>y</sub> protective film which is formed with 25sccm nitrogen flow not only has the environmental adaptability as the SiO<sub>x</sub> film but also resists to the salt spray corrosion. But the average reflectivity of the SiO<sub>x</sub>N<sub>y</sub> protected aluminum coating decreases about 2%, reach 88.6%, in the band 400nm-2500nm.