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Although it is generally accepted by the perovskite solar cells community that their interface behaviors have a profound impact on their power conversion efficiency (PCE) and stability, such interfacial engineering on the bottom interface between electron transport layer (ETL) and perovskite film is still lagging. Herein, a 3D graphene (G)–CuInS2 composite is designed as an efficient ETL to improve both the interfacial contact and passivate defects at the G–CuInS2/perovskite interface. The lattice matching of graphene and methylammonium lead iodide CH3NH3PbI3 inhibits the concentrated stress generated during the growth of perovskite, resulting in crystal films with large grain boundaries. The low‐electron defect density in 3D G–CuInS2 composite facilitates the electron transport from perovskite film to CuInS2 quantum dots. In addition, 3D G–CuInS2 shows excellent carrier extraction capability of reducing carrier extraction time by 1.47 times than that of the counterpart. Correspondingly, a highly improved PCE of 22.4% is obtained, which increases by 15% of the counterpart. Furthermore, the unencapsulated device based on 3D G–CuInS2 shows long‐term stability, maintaining 85% of its original efficiency in air for 30 days. This strategy provides a new route to interfacial passivation engineering for preparation of high‐performance perovskite solar cells. [ABSTRACT FROM AUTHOR]