Quasi-two-dimensional perovskite light-emitting diodes (quasi-2D PeLEDs) are emerging as high-potential candidates for new generation of wide-color gamut displays due to their simple, low-cost solution process, and high color purity. However, the luminescence performance of quasi-2D perovskite films is severely limited by dispersed phase distribution and excessive defect density, which are caused by excessive diffusion of nucleation sites during the perovskite growth stage. Here, the benzylphosphonic acid (BPA) molecule, owing to its strong P–O–Pb bond energy sites and strong electronegativity to PEA⁺, can aggregate lead-halide octahedron to grow high-dimensional phases, avoiding scattered low-dimensional phases (n = 1). The continuous gradient phase distribution will be beneficial to smooth carrier injection and effectively suppress the leakage current in PeLEDs. Meanwhile, the introduction of phosphonic acid groups will fill the vacancies of Pb ions and reduce non-radiative recombination. As a result, the maximum external quantum efficiency (EQE) of PeLEDs can be increased from 8% to 20.6% with a 514 nm light emission and a 21 nm full-width half maximum, and the device lifetime (T₅₀) is nearly 6-fold of the pristine sample. In addition, this strategy is also suitable for other wavelength. For example, in blue light, performance improvement is also realized that the maximum EQE of 8% and the luminance increased from 1045 to 5264 cd/m². These results provide a feasible strategy to regulate the phase distribution and passivate the defects of quasi-2D perovskites.