Although the power conversion efficiency (PCE) of thermally stable inorganic CsPbIBr2 perovskite solar cells (PSCs) is over 10%, the severe interfacial and nonradiative recombination deteriorates the open‐circuit voltage (Voc). Herein, an ultrathin wideband MgO is mediated between the SnO2 electron transport layer (ETL) and the CsPbIBr2 photoabsorber to passivate the undesirable recombination, thereby enhancing the Voc. Meanwhile, the δ‐phase perovskite located at the interface between SnO2 ETL and CsPbIBr2 film is reduced after MgO modification, because the MgO layer provides a substrate for perovskite growth and reduces vacancy. Moreover, the tunneling effect and better band alignment effectively block holes and accelerate electrons to the electrode. Consequently, for optimal MgO‐modified devices, a shining improvement of Voc from 1.25 to 1.36 V without short‐circuit current losses boosts the champion CsPbIBr2 PSCs to obtain a PCE of 11.04%, which is the highest value among the pure‐CsPbIBr2 PSCs. However, the MgO layer significantly reduces severe hysteresis and increases device stability.