SnO2 is considered as a promising electron transport material for carbon-based perovskite solar cells (C-PSCs) due to its excellent electron mobility, stability over TiO2, and low-temperature processing. However, the lattice mismatch and poor contact quality of the SnO2/CH3NH3PbI3 interface, as well as oxygen vacancies, usually lead to nonradiative recombination and limit the further improvement of photovoltaic performance. In this paper, two chitosan derivatives, chitosan quaternary ammonium salt (HACC) and carboxymethyl chitosan (CMCS), were used as additives for SnO2 to adjust the energy level and improve the contact performance of the SnO2/CH3NH3PbI3 interface. These two additives have suitable terminal active functional groups, amino group and hydroxyl group, which can interact with SnO2 and CH3NH3PbI3 at the interface, induce the crystal growth of the perovskite, and play a good role in passivating interfacial defects. Therefore, the interfacial contact and the charge-transfer ability were effectively enhanced. As a result, the addition of HACC and CMCS increased the PCE of C-PSCs from 10.17 to 12.42 and 13.39%, and the repeatability and long-term stability of the corresponding unencapsulated C-PSCs were also significantly improved. This work expands the vision for the future interface modification strategies to improve the device performance and is conducive to promote the further commercialization of C-PSCs.