Quasi-2D metal halide perovskites have emerged as highly promising candidates for the emissive layer in low-cost and high efficiency light emitting diodes (LEDs). Despite tremendous efforts have been made to improve the performance of quasi-2D perovskite LEDs (PeLEDs), previous studies have mainly focused on enhancing the radiative recombination efficiency via phase distribution control or defect passivation of perovskite films. However, an in-depth understanding of the crystallization kinetics of quasi-2D perovskites, crucial for achieving high performance PeLEDs, is still lacking. Herein, we propose the introduction of a strategically designed intermediate phase to regulate the crystallization behavior of quasi-2D perovskite films. It is revealed that the intermediate phase of phenylphosphonic acid (PPA)-PbI composite can significantly lower the nucleation energy barrier, effectively manipulating the crystallization kinetics and drastically improving the overall quality of the perovskite films. As a result, the PPA-treated quasi-2D PeLEDs obtain a peak external quantum efficiency (EQE) of 22.74 % along with an ultrapure red emission at 650 nm with CIE coordinates of (0.708, 0.2919), which is among the closest point approaching the pure-red light coordinates of (0.708, 0.292) in display standard of REC.2100 reported to date. Meanwhile, the optimal devices show excellent spectral stability and prolonged operational lifetime compared to control devices. This work presents a novel approach to constructing high-performance quasi-2D PeLEDs by modulating the crystallization kinetics of perovskite films, opening new avenues for future advancements in this field.