The technology for coating protective dielectric layer to 2.4 m telescope′s aluminium coating is studied using the large vacuum evaporation coating machine ZZS3200, which is the first vacuum evaporation chamber evaporating materials from top to bottom in China. According to the profile of 2.4 m telescope′s primary mirror and the geometry of ZZS3200, the uniformity of the dielectric layers is analyzed and a moly boat source for evaporating materials from top to bottom is designed. The result shows that the index of evaporation characteristics is approximate to 1, and the non-uniformity of the dielectric film on 2.4 m mirror is less than 13.6%. Comparing the effect of removing the old protected aluminium coating, it is found that, compared with the SiO2-protected coating, MgF2-protected coating is easier to be removed, which can decrease the erosion of mirror during the coating removal, and has higher speculum reflectivity. The 2.4 m telescope′s mirror aluminium coating with MgF2-protected layer is completed. The reflectivity of witness sample is 87.16％ in the wavelength range of 350～1100 nm, and the limiting magnitude is not less than 23.5 magnitude.