The invention discloses a method for preparing bismuth oxide. The method comprises the following steps: adding a 400g/L bismuth nitrate solution into a reaction kettle, stirring, adding a 300g/L sodium hydroxide solution, and regulating the PH to be 1.5; standing until the solution is completely precipitated, and pumping the supernatant to be dry; adding purified water for stirring, adding a 150g/L sodium hydroxide solution, regulating the PH to be 5, and adding polyethylene glycol; heating and raising the temperature to be 65 DEG C; adding a 150g/L sodium hydroxide solution, regulating the PH to be 12, and adding a 300g/L sodium hydroxide solution to regulate the mass concentration of the sodium hydroxide to be 2%; heating to 85 DEG C within an hour, and performing heat preservation for 6 hours; centrifuging and washing; drying at the temperature of 200 DEG C until the moisture content is less than 0.1%. According to the preparation method, high temperature is not needed, the operation is simple, byproducts which pollute the environment are avoided, the obtained bismuth oxide is a needle product, and the product is high in purity.