Despite the rapid progress of perovskite materials in emerging perovskite photovoltaic devices, they still suffer from the polycrystalline nature associated with grain boundaries (GBs) which are vulnerable to moisture permeation and/or ion migration. Besides, charge carrier recombination of GBs through defect states plays a crucial role in restricting the performance and stability of perovskite photovoltaics. To address such detrimental issues, quinary kesterite nanocrystals, namely Cu2NiSn(S,Se)4 (CNTSSe), having narrow size distribution below 10 nm by a facile hot-casting method are rationally designed and employed as a passivation agent for the GBs/surface of perovskite films. This passivation strategy greatly reduces defect states at perovskite GBs and promotes continuity between adjacent grains, resulting in accelerated hole transport ability and suppressed interfacial recombination. Thereupon, champion power conversion efficiency of 20.8% (20.5 ± 0.3% in average) (Cs0.05(FA0.90MA0.10)0.95Pb(I0.90Br0.10)3), 18.9% (MAPbI3), and 18.7% (FAPbI3) is achieved with a negligible hysteresis and outstanding stability by retaining over 85% of initial performance under ambient conditions with continuous illumination over 900 h. Herein, not only a universal approach to effectively passivate the GBs of the perovskite films by inorganic nanocrystals is presented, but also a deep understanding of detrimental defects on the photovoltaic performance and stability of perovskite solar cells is ensured.