Potassium-ion batteries (PIBs) are expected to replace lithium-ion batteries (LIBs) as the next generation of electrochemical energy storage products. However, due to the large radius of potassium ion, the anode material undergoes drastic volume expansion during the battery cycle, resulting in performance degradation. Bi2O3 is a kind of material with high theoretical specific capacity, but there is seldom research in potassium-ion batteries. Herein, we design a Bi2O3 nanomaterial embedded in a carbon matrix. The introduction of carbon matrix can not only improve the electronic conductivity of the material, but also alleviate the volume expansion of bismuth oxide. After 100 cycles of 50 mA g−1, its specific charging capacity is 314 mA h g−1, which shows good cycle stability. By ex situ X-ray diffraction, cyclic voltammetry and charge-discharge curves, it is confirmed that the mechanism of potassium ion storage on Bi2O3@C electrode is the combination of transformation reaction mechanism and alloying reaction mechanism.