Abstract: The technology for coating the telescope primary mirror with protected aluminium layer is studied during the preparative process, when the mirror is face up. According to the geometry of ZZS3200 large coating equipment, the traditionally calculative model is modified, and the uniformity of the aluminium and dielectric layers is analyzed separately. The simulated results accord with the experimental results, and in the 2.4 m diameter range, the non-uniformity of the whole film is within 5.5%. The 1.8 m-diameter plane reflective primary mirror has been prepared with optimized technical parameters. Its reflectance after coating is 88.6% in the wavelength range from 350 to 1 800 nm. Furthermore, the films have passed the environment stability test successfully.