In the past several years, metal-halide perovskites have attracted much attention for their promising applications in light-emitting diodes (LEDs). Significant progress has been made in the development of high-efficiency green, red, and near-infrared perovskite LEDs (PeLEDs), but blue PeLEDs are still lagging behind. Although the external quantum efficiency (EQE) of blue PeLEDs has exceeded 10%, these works mainly focus on pure blue and sky-blue regions. Creating deep-blue electroluminescence (EL) with high stability, efficiency, and color purity is still a great challenge for PeLEDs. Herein, deep-blue PeLEDs are reported which have been constructed with a mixture of a quasi-2D phase and a nanocrystal phase by introducing a small diamine cation, ethylene diammonium (EDA²⁺), and the effect of EDA²⁺ on quasi-2D perovskite crystallization is investigated. EDA²⁺ with two amino ending groups has a stronger inhibitory effect on high-n phase growth than phenylethyl ammonium, leading to the formation of small nanocrystals. Further introduction of Na⁺ reduces the size of the nanocrystals and broadens the band gap. Based on these improvements, deep-blue PeLEDs with EL peaks at 467 (EQE ≈ 1.7%) and 461 nm (EQE ≈ 1.1%) are successfully constructed. Thus, this study provides a useful strategy for improving the performance of deep-blue PeLEDs.