The invention discloses a method for synthesizing bismuth trioxide-cuprous oxide nano-composite through a mechanical milling thermal treatment two-step method. The method comprises the following steps: mixing solid phase raw material sodium bismuthate dihydrate, basic cupric carbonate and oxalic acid dihydrate, performing mechanical milling solid phase reaction to acquire Bi2O2CO3-Cu2(OH)2-2x(CO3)xC2O4 compound precursor; and performing thermal treatment on the precursor to obtain the Bi2O3-CeO2 nano-composite. Solid mixing, the raw material room-temperature milling solid-phase reaction and thermal treatment two-step method are adopted, the preparation process is simple and easy to control, and the agglomeration of product particles can be greatly reduced; an operation of furthermore adding oxidation reducing agent, the template agent and the solvent is avoided, the product purity is improved, the green synthesis requirement of the material is satisfied, and the method is suitable forthe large-scale production.