The invention discloses a metal flange coated with an insulating ceramic coating and a preparation method thereof. The insulating ceramic coating comprises the following material components in percentage by weight: 20 to 50 percent of TiO2 and the balance of Al2O3. The coating position comprises all outer surfaces of a metal flange substrate. The coating has a thickness between 0.1 and 0.6mm, a surface roughness less than or equal to 3.2mu m and a porosity less than 15 percent. The roughness of the substrate is not more than 3.2mum. The bonding strength of the coating and the substrate is more than or equal to 10MPa. All the angularity blend radius of the metal flange substrate is more than or equal to 1.0mm. The angularity radius of the coating is consistent with the round radius of the substrate. The invention makes the heterogeneous flange in a reliable electrical insulation state, and prevents the electrochemical corrosion as well as improves the work temperature of the flange from about 300 DEG C to about 900 DEG C.