The invention relates to magnesium alloy surface treatment technology, in particular to highly corrosion-resistant ceramic coating solution prepared through magnesium alloy surface micro-arc oxidation and application thereof. The defects that a magnesium alloy workpiece has a relatively rough surface after being subjected to micro-arc oxidation treatment in fluorozirconate and is severely corroded in oxidizing solution are overcome. 1L of micro-arc oxidizing solution consists of 6 to 12g of fluorozirconate, 2 to 10g of dihydric phosphate, 2 to 8g of fluoride and 2 to 10g of citrate. A method for preparing a composite ceramic coating by using the solution comprises the following step of: treating a magnesium alloy workpiece subjected to pretreatment and the conventional micro-arc oxidationtreatment respectively in the micro-arc oxidizing treatment solution to obtain the composite ceramic coating. Compared with the conventional magnesium alloy micro-arc oxidation coating, the compositeceramic coating has higher corrosion resistance, uniform thickness and low surface roughness, is compact and does not need aftertreatment such as sealing and the like; and the solution has simple components and is suitable for industrial production, and the raw materials are readily available.