The service life of the large primary mirror with aluminum coating can be effectively prolonged by a protective layer. The SiOx (1<x<2) protective material which is thermally evaporated from up to bottom is studied. Environmental adaptability experiment, spectral measurement and micro-morphology analysis are executed on bare SiOx coatings within different oxygen concentrations, and the repeatability verification is implemented by testing the SiOx protected aluminum coatings. The results show that the SiOx coatings can meet the protective qualification within oxygen flow of 50 sccm (p=5.6 \times 10-3 Pa). The fine compactness of the coating has excellent moisture resistance with the average molecular spacing of 0.335 nm. In addition, the average reflectivity of the SiOx protected aluminum coating is 90.47% in the band 400 -2 500 nm and the coating is confirmed to have good environmental performance.