The conversion of carbon dioxide has become a hot topic in the world today. Here, we adopt the strategy of in-situ electrochemical transformation to reduce layered bismuth oxide formate nanoflowers (BiOCOOH NFs) self-assembled with nanosheets synthesized by simple solvothermal method to porous bismuth nanoflowers (p-Bi NFs) with a large number of lattice dislocations. Specifically, 1.0 g Bi(NO₃)₃•5H₂O was ultrasonically dissolved in 10 mL N,N-dimethylformamide (DMF), then 70 mL deionized water was added to the above solution, and the resulting solution was ultrasonicated for 10 min at room temperature to ensure that all reagents were uniformly dispersed. The resulting solution was then transferred to a 100 mL Teflon-lined stainless steel autoclave, kept at 120 ℃ for 20 h, and then naturally cooled to room temperature. The results show that the minimum overpotential of the electrochemical reduction of carbon dioxide to formate is 436 mV. When the catalyst loading is 0.5 mg/cm², the partial current density of formate (j_formate) is as high as 24.4 mA•cm^-2, which is 5.5 times that of commercial bismuth (Commercial Bi); and the Faraday efficiency (FE_formate) of formate is 96.7% at –1.8 V versus saturated calomel electrode (vs. SCE). The FE_formate is over 90% in a wide potential window of over 500 mV. Moreover, the p-Bi NFs electrocatalyst is stable in formate production for more than 10 h in CO₂-saturated 0.5 mol•L^-1 KHCO₃ electrolyte. Compared with the normalized electrochemical surface area (ECSA), it was found that the j_formate of p-Bi NFs was still about 4.5 times higher than that of Commercial Bi. The high catalytic performance of the catalyst can be attributed to the unique micro/nano hybrid structure derived from the lattice collapse and reconstruction of precursors, resulting in porous and rough surface and containing high density of active sites with lattice dislocations and defects. This study provides new insights into designing and synthesizing electrocatalysts with high performance for carbon dioxide reduction to formate.