The development of highly efficient robust electrocatalysts with low overpotential and industrial-level current density is of great significance for CO2 electroreduction (CO2ER), however the low proton transport rate during the CO2ER remains a challenge. Herein, a porous N-doped carbon nanofiber confined with tin-nitrogen sites (Sn/NCNFs) catalyst is developed, which is prepared through an integrated electrospinning and pyrolysis strategy. The optimized Sn/NCNFs catalyst exhibits an outstanding CO2ER activity with the maximum CO FE of 96.5%, low onset potential of −0.3 V, and small Tafel slope of 68.8 mV dec−1. In a flow cell, an industrial-level CO partial current density of 100.6 mA cm−2 is achieved. In situ spectroscopic analysis unveil the isolated Sn-N site acted as active center for accelerating water dissociation and subsequent proton transport process, thus promoting the formation of intermediate *COOH in the rate-determining step for CO2ER. Theoretical calculations validate pyrrolic N atom adjacent to the Sn-N active species assisted reducing the energy barrier for *COOH formation, thus boosting the CO2ER kinetics. A Zn-CO2 battery is designed with the cathode of Sn/NCNFs, which delivers a maximum power density of 1.38 mW cm−2 and long-term stability.