Subhani et al. found that Sm-doping in CsPbIBr2 decreased its bandgap from 2.05 eV to 1.8 eV; thus, the efficiency of CsPbIBr2 solar cells was improved by ∼30%. However, Sm is a vital strategic resource with high costs. Metal Sn is much more abundant and cheaper than Sm; meanwhile, it has been proven that Sn can adjust the bandgap of CsPbIBr2 in a broader range, 2.05 eV to 1.64 eV. Therefore, Sn-doping in CsPbIBr2 may improve the efficiency of CsPbIBr2 solar cells, even to a greater extent. In this work, we established the TiO2/CsPbIBr2 interface model by gradient Sn-doping in CsPbIBr2 and investigated the impacts of such gradient doping on the carrier separation behaviors at the TiO2/CsPbIBr2 interface from the aspects of the cross-interface electric field, bandgap, and band matching, based on first-principles calculations. It is found that gradient Sn-doping can transfer more electrons from TiO2 to perovskites, thus creating an enhanced cross-interface electric field conducive to the separation of carriers at the TiO2/CsPbIBr2 interface. Affected by the existence of the interface, the bandgap of each perovskite layer gradually increases as it moves away from the interface; in addition, due to the gradient Sn-doping, the steps between the bandgaps of adjacent perovskite layers become smaller and more uniform, which is favorable for the separation of electrons. In summary, gradient Sn-doping can improve the carrier separation at the TiO2/CsPbIBr2 interface.