Perovskite nanograins with a dimension larger than the Bohr exciton diameter have significant advantages in achieving high-performance light-emitting diodes (LEDs) due to their bandgap uniformity and strong charge confinement effect. However, perovskite nanograin films prepared by the solution spin-coating method are prone to produce massive defects due to the fast crystallization rate, leading to severe nonradiative recombination that greatly detracts the performance of LEDs. Therefore, regulating crystallization and minimizing defects plays a crucial role in the development of perovskite nanograins for high-performance LEDs. Herein, a simple dual-additive strategy is reported to manipulate the growth of high-quality CsPbBr-based nanograin films. A multifunctional additive 5-aminovaleric acid (5AVA) is introduced to slow down the crystallization rate of CsPbBr, followed by the further addition of triphenylphosphine oxide (TPPO) to achieve effective synergistic passivation of defects, which can significantly enhance radiative recombination rate and reduce defects-induced nonradiative recombination. Ultimately, based on the 5AVA and TPPO co-modified CsPbBr nanograins, the perovskite LEDs are fabricated achieving a maximum external quantum efficiency (EQE) of 20.95% and an average EQE of approaching 20%, demonstrating excellent reproducibility. This work provides new insight into the crystallization regulation and defect passivation of perovskite nanograins and the further improvement of device performance.