Solution-processed Perovskite Light-Emitting Diodes (PeLEDs) has drawn much attention due to their low cost, narrow emission spectra and wide color gamut. However, undesirable pinholes and defects of perovskite films impair device performance, which is attributed to the atactic nucleation and rapid crystallization during solution processing. In general, the antisolvent is added to the perovskite spin coating process to quickly remove excess solvent and form an intermediate phase. Combining with annealing treatment, mesophase gradually transforms into the perovskite phase and dense perovskite film can be obtained. At the same time, the larger organic cation (PEA+, TEA+, etc.) will cut continuous lead halide octahedron into the periodic quantum well structure to form quasi-two-dimensional perovskite which has a larger exciton binding energy and rapid energy transfer.By adding the large polymers to the precursor, long segments cannot enter the perovskite lattice and act as the “framework” in the perovskite crystallization process, which can restrict nucleation sites, inhibit the rapid growth of grains and further optimize the film quality. However, the size, fluidity and solubility of the polymer change with the increase of the segment length, which significantly affects the growth of perovskite crystals. Therefore, it is crucial to systematically study the regulation law of polymer segment length on the crystallization kinetics of perovskite to improve the quality of perovskite thin films and thus promote the electroluminescence properties.Polyvinyl Pyrrolidone (PVP), a common non-ionic polymer, its structure is relatively simple and carbon segment length can be accurately controlled by molecular weight. Moreover, PVP contains a carbonyl group, which can effectively passivate the defects in perovskite. In this work, perovskite crystallization kinetics has been tuned by the incorporation of Polyvinyl Pyrrolidone (PVP) with different segment length. Based on in-situ photoluminescence and X-Ray Diffraction (XRD) spectra, it showed that perovskite crystallization rate is retarded to inhibit small n phase formation by increasing the PVP segment length, which played an important role in determining perovskite film quality, such as crystallinity, charge carrier recombination and roughness, etc. After different segment PVP was added to the precursors, the absorption peaks at 403 nm(n=1), 434 nm(n=2) and 465 nm(n=3) in the Ultraviolet-Visible Spectrum of the quasi-two-dimensional perovskite low-dimensional phase are significantly inhibited, which are considered adverse to Photoluminescence performance. According to space charge limited current tests, compared with films without additives, the defect density of the films decreased significantly after adding different segment PVP, reaching the minimum at PVP-10 kDa. And with the segment length continuing to increase ($>$10 kDa), the defect density will increase slightly. The short PVP segment (≤3 kDa) was not effective on adjusting the crystallization rate, resulting in minor improvement in roughness and photoluminescence quantum yield (PLQY) of the perovskite film. While the too-long PVP segment (≥30 kDa) inhibited crystallization, leading to high roughness, low crystallinity and deteriorated PLQY of the perovskite film. 1With the suitable PVP segment length (~10 kDa), the perovskite crystallization kinetics can be optimized, which exhibited decreased roughness from 1.489 nm to 0.954 nm, decreased defect density from $\mathrm{1.55}\times {\mathrm{10}}^{\mathrm{18}}$ cm^-3 to $\mathrm{1.05}\times {\mathrm{10}}^{\mathrm{18}}$ cm^-3, improved PLQY from 16.9% to 58.9%. Combined with superior morphology and PLQY of the perovskite films with PVP-10 kDa, it is promising to achieve a high electroluminescence performance. The PeLEDs before and after incorporation of PVP-10 kDa were prepared with a stacking structure ITO/ PVK/Perovskite/TPBi/LiF/Al. Compared with the control devices, the PVP-10 kDa devices showed little change in luminance but significantly reduced current density, which benefited improved maximum external quantum efficiency (EQE) from 8.55% to 18.00%. This work systematically investigated polymer segment length dependent perovskite crystallization and electroluminescence performance, which provides a guideline to design polymer additives to further promote the development of PeLEDs.