The invention relates to methods for treating magnesium alloy surfaces, in particular to nickel-phosphorus nanometer silicon dioxide composite plating of magnesium alloy and a method for preparing the nickel-phosphorus nanometer silicon dioxide composite plating. The method includes mechanical polishing, oil removing, acid washing and alkali washing pretreatment. Chemical plating includes two steps implemented by the aid of fluorine-containing plating solution A without nanometer silicon dioxide and nanometer-silicon-dioxide-containing plating solution B without fluorine. The nickel-phosphorus nanometer silicon dioxide composite plating and the method have the advantages that composite nanometer silicon dioxide is added into the plating solution B, so that the corrosion resistance of the nickel-phosphorus nanometer silicon dioxide composite plating can be enhanced; preparation of nanometer oxide concentrated solution or pretreatment on the nanometer silicon dioxide can be omitted, inexpensive nickel sulfate hexahydrate is used as major salt, only the fluorine-containing plating solution is applied at the first step in plating processes, the plating solution for the second step does not contain fluoride, accordingly, the usage of fluorine can be effectively reduced, and the cost can be lowered.