A method of preparing bismuth oxide and an apparatus therefor are disclosed. The method includes: melting bismuth metal; transporting the melted bismuth metal to an open first reactor and oxidizing the melted bismuth metal while stirring at a temperature of 300-650° C.; and transporting bismuth oxide and un-reacted material to a closed second reactor through a screw and oxidizing the bismuth oxide and un-reacted material while rotating the closed second reactor at a temperature of 300-600° C. with a supply of air or oxygen.