Formate, as an important chemical raw material, is considered to be one of the most promising products for industrialization among CO electroreduction reaction (CORR) products, but it still suffers from poor selectivity and a low formation rate at a high current density on account of the competitory hydrogen evolution reaction. Herein, the heterogeneous nanostructure was constructed by anchoring InO nanoparticles on poly(3,4-ethylenedioxythiophene) (PEDOT)-modified carbon black (InO/PC), in which the PEDOT polymer interface layer could immobilize InO nanoparticles and obtain a notable reduction in electron transfer resistance among the InO particles, showing a 27% increase in the total electron transfer rate. The optimized InO/PC with rich heterogeneous interfaces selectively reduced CO to formate with a high FE of 95.4% and a current density of 251.4 mA cm under -1.18 V vs RHE. Also, the formate production rate for InO/PC was up to 7025.1 μmol h cm, surpassing most previously reported CORR catalysts. The in situ XRD results revealed that InO particles were reduced to metallic indium (In) as catalytic active sites during CORR. DFT calculations verified that a strong interface interaction between In sites and PC induced electron transfer from In sites to PC, which could optimize the charge distribution of active sites, accelerate electron transfer, and elevate the p-band center of In sites toward the Fermi level, thereby lowering the adsorption energy of *OCHO intermediates for CO conversion to formate.