Ion defects at surface and grain boundaries (GBs) of perovskite films are paramount factors that influence the power conversion efficiency (PCE) of organic–inorganic hybrid perovskite solar cells (PSCs). Various methods have been proposed to solve the problems, but it still remains a great challenge because of the sophisticated and multiplicity of these defects. Herein, a modification method is developed that all‐inorganic low‐dimensional cesium copper halide nanocrystals (Cs3Cu2I5 NCs) are introduced to reduce the ionic defects of perovskite films at the surface and GBs. The champion device modified by Cs3Cu2I5 NCs exhibits remarkable promotion of PCE from 19.88% to 22.03% and fill factor from 74.09% to 82.21%. The results show that the solid‐state interdiffusion process occurs between the perovskite films and Cs3Cu2I5 NCs, which can improve the crystallinity, reduce interfacial recombination loss, and enhance the interfacial carrier transport in PSCs. Especially, the element distribution of Cs3Cu2I5 NCs in perovskite films affects the effectiveness of ionic defects passivation in the perovskite lattice. The PSCs device retains 91.6% of its initial PCE after 504 h under high moisture conditions. This work demonstrates an interfacial engineering strategy using the characteristic perovskite NCs, which provides a passivation method to achieve the high‐performance PSCs.