Highly active catalysts are in great demand for CO2 electroreduction (CO2ER) due to the kinetically sluggish nature of CO2. Herein, a novel oxygen vacancy (Vo)-rich In-SnO2 hollow nanofiber catalyst is developed to combine the advantages of tuning both morphological and electronic properties. In-doped SnO2 grain-stacked hollow nanofiber morphology is fabricated through a simple electrospinning method, aiming to enlarge the surface area for hosting Vo, as well as facilitate long-range charge and mass transfer. In-doping synergistically enhances catalytic activity by suppressing SnO2 grain size and increasing electron density on Sn. Superficial Vos on metal oxide are effectively generated and utilized through an in situ pre-electroreduction process successively with CO2ER, which tailor surface electronic properties by stabilizing the reduction intermediate CO2 center dot- in an O-coordinated manner and provide interpretation for high activity of "oxide-derived" metals. The Vo-rich In-SnO2 hollow nanofiber catalyst, assembled with an efficient three-phase interface established by an electrochemical hydrogen pump reactor, exhibits excellent overall performance with considerable HCOOH Faradaic efficiency of about 86.2% and partial current density of about 28.5 mA cm(-2) at -1.34 V versus reversible hydrogen electrode and is at the top-level as compared with other Sn-based catalysts reported recently.