Electroreduction of CO2 into liquid fuels is an appealing strategy to alleviate energy and environmental crisis. However, the poor product selectivity and insufficient energetic efficiency of current catalysts for CO2 electroreduction severely hinder their widespread applications. Herein, a partially reduced Sn/SnO2 porous hollow fiber (PHF) is, for the first time, proposed to electroreduce CO2 into formate with high selectivity, efficiency and durability. The Sn/SnO2 PHF is synthesized via a facile combination of electrospinning and reduction process. The partial reduction can efficiently tune the electronic structure of Sn and create abundant oxygen vacancies, and consequently tailors the electrocatalytic ability. Importantly, Sn/SnO2 PHF demonstrates a high formate faradaic efficiency of 82.1% and a considerable formate partial current density of ∼22.9 mA cm−2, favorably rivals most state-of-the-art catalysts for electroduction CO2 into formate. This study provides new insights into exploring efficient catalysts for CO2 electroreduction, and it suggests that the rational design of metal/metal oxide heterostructure shows great potential as a promising type of electrocatalysts.