Perovskite light-emitting diodes (PeLEDs) have been developed at a rapid pace with high external quantum efficiencies (EQEs) surpassing 20% for red, infrared, and green emissions; nevertheless, the performance of their blue-emitting cousins remains relatively quite low. Among various crucial factors that could impact these solution-processed devices, selection of a suitable underlayer (usually a hole-transport layer) is critical to the quality of resulting perovskite films. Particularly, the role of the substrate is highly important in the case of quasi-2D perovskite films where the emission occurs as a result of cascade energy transfer among various perovskite domains with different numbers of lead-halide octahedral layers (n) sandwiched between long-chain organic molecules. Herein, we explored the impact of the substrate on domain distribution of quasi-2D perovskite films, and a series of efficient and color-stable blue PeLEDs is attained through precise stoichiometric engineering. By employing a range of characterization tools, we thoroughly investigated the crystallization mechanism of quasi-2D perovskites on different substrates, including poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)- co -(4,4′-( N -(4-sec-butylphenyl)diphenylamine)] (TFB). Perovskite films spin-coated on PEDOT:PSS exhibited cyan emission from then≥ 4 quasi-2D phase with uniformly distributed small grains. In contrast, perovskite layers formed on TFB featured an island morphology and showed blue emission from the quasi-2D phase withn-values between 2 and 4. In addition, we examined how the variation of A-site cations affects the optical properties of quasi-2D perovskites. Consequently, PEDOT:PSS-based champion PeLED exhibited cyan emission at 495 nm with a maximum EQE of 6.73%, whereas TFB-based counterparts manifested blue emission at 486 nm with a peak EQE of 3.95%. We anticipate that our work will provide a fundamental guideline to rationally selecting the substrates for the optimized growth of quasi-2D perovskites toward high-efficiency PeLEDs.