The invention discloses a method for preparing a bismuth titanate nano-composite oxide through the room-temperature solid phase reaction. The method is characterized in that the solid phase materialsof dihydrate sodium bismuthate, sodium hydrogen sulfite and titanium sulfate are mixed, the ball-milling solid phase reaction is conducted, and an obtained product is washed, centrifuged, separated and dried to obtain the Bi12TiO nano-composite oxide, that is, the target product. The solids are mixed, the raw materials are subjected to the ball-milling solid phase reaction at the room temperature,the preparation process is easy to operate, is easy to control, and greatly reduces the hard aggregation of the product particle; the additional addition of an oxidation reducing agent, a templatingagent and a solvent is avoided, the product purity is increased, the green synthesis requirement of the material is met, and the method is also suitable for large-scale production.