The electrochemical CO₂ reduction to formate represents a promising route for carbon neutrality. However, current bismuth (Bi)-based catalysts suffer from limited active site exposure, poor charge transfer kinetics, and rapid performance degradation. Herein, we report a three-dimensional (3D) carbon fiber aerogels (CFAs) with a porous network architecture supporting well-distributed Bi nanoparticles (BiNPs) that synergistically address these challenges via geometrical structure and interfacial electronic modulation. The 3D porous network of the catalyst offers a high specific surface area of 534.39 m²·g⁻¹, promoting CO₂ adsorption and efficient charge transport. Electrochemical characterization reveals that the 0.02 M BiNPs@CFAs catalyst achieves a Faradaic efficiency of formate (FE_formate) of 96.73% ± 1.45% at −1.0 V vs. reversible hydrogen electrode (RHE), while sustaining a high partial current density of −221.7 mA∙cm⁻² in the flow-cell operation. After continuous operation for 72 h, the FE_formate values remained above 90.6%. In-situ electrochemical Fourier transform infrared (FTIR) spectroscopy and the density functional theory (DFT) calculations confirm that Bi active sites effectively stabilize the $\text{*}$OCHO intermediate, a key step in formate formation. This work establishes new ideas for designing efficient and stable electrocatalysts through synergistic structural and electronic modulation by metal nanoparticles and aerogel architectures.