The electrochemical reduction reaction of CO2 provides a renewable method to close the carbon cycle and alleviate the global energy issue. Bi-based electrocatalysts present huge prospects for catalyzing formate-selective CO2 reduction. Herein, we fabricated a porous vegetable sponge-like bismuth oxide (VS-Bi2O3) for the selective electroreduction of CO2 to formate, which underwent in situ reconstruction to form 2D nanosheets containing metallic Bi and Bi2O2CO3. We propose that the unique porous morphology and low crystallinity of VS-Bi2O3 are beneficial for the in situ generation of Bi2O2CO3, which might play a significant role in enhancing the CO2 electrocatalysis performance. The catalyst delivers a 93.7% faradaic efficiency of formate at 400 mA cm−2 and shows stability for over 18 h at 100 mA cm−2.