Organic‐inorganic halide perovskite solar cells (PSCs) have drawn tremendous attention owing to their remarkable photovoltaic performance and simple preparation process. However, conventional wet‐chemical synthesis methods inevitably create defects both in the bulk and at the interfaces of perovskites, leading to recombination of charge carriers and reduced stability. Herein, a bilateral interface modification to perovskites by doping room‐temperature synthesized CsPbBr3 nanocrystals (CN) is reported. The ultrafast transient absorption measurement reveals that CN effectively suppresses the defect at the SnO2/perovskite interface and boosts the interfacial electron transport. Meanwhile, the in situ Kelvin probe force microscopy and contact potential difference characterizations verify that the CN within the upper part of the perovskites enhances the built‐in electric field, facilitating oriented migration of the carriers within the perovskite. Combining the superiorities of CN modifiers on both sides, the bilaterally modified CH3NH3PbI3‐based planar PSCs exhibit optimal power conversion efficiency exceeding 20% and improved device stability. Room‐temperature synthesized CsPbBr3 nanocrystal (CN) is exploited as the bilateral interface modifier in perovskite layer for efficient planar perovskite solar cells (PSCs). Owing to the CN induced bilateral interfacial passivation and boosted built‐in electric field, the charge separation and transfer are significantly ameliorated, which contribute to the superior power conversion efficiency exceeding 20% in CH3NH3PbI3‐based planar PSCs.