The work aims to obtain the modified micro-arc oxidation process by optimizing the electrolyte formula and electrical parameter output mode to prepare SiO2 and Al2O3 composite ceramic coating with excellent comprehensive properties. Alkaline silicate electrolyte was used as the arc initiator and 80~150 mL/L alkaline silica sol solution and 60~80 g/L sucrose were used as auxiliary additives to prepare SiO2 and Al2O3 composite ceramic coatings on 7A55 aluminum alloy by adding negative electrical parameters. Microstructure and elements of the composite coating were studied by test methods like SEM and EDS. The mechanism of membrane formation by new process was studied by comparing the micro-arc oxidation process used to prepare single Al2O3 ceramic coating. SiO2 and Al2O3 composite ceramic coating with thickness over 100 μm could be prepared on 7A55 aluminum alloy by modified process and the preparation efficiency was increased by 2~3 times when compared with the micro-arc oxidation process used to prepare single Al2O3 ceramic coating. The content of SiO2 and Al2O3 in the composite coating respectively decreased and increased from the coating surface to the inner side. The composite ceramic coating showed a compact surface in the form of spherical particles without cracks or micro pores which existed in the single Al2O3 ceramic costing prepared by MAO. In addition to forming micro-area plasma arc sintering by micro-area discharge to obtain Al2O3 ceramic coating, the new process also had SiO2 adsorption deposition, which was a comprehensive process of adsorption and sintering. Adding alkaline silica sol containing nano-SiO2 to modify electrolyte is beneficial to improving the structure and compactness of ceramic coating.