The work aims to improve optimal energy density process windows of excimer laser crystallization and enhance grain size uniformity of polycrystalline silicon, and finally improve characteristics of low temperature poly silicon thin film transistor (LTPS TFT). Amorphous silicon (a-Si) films of different thickness and refractive index were deposited on glass substrates by adopting PECVD technology. After dehydrogenation in high temperature annealing furnace, excimer laser annealing was performed to complete transition from amorphous silicon to polycrystalline silicon (P-Si). Grain size and surface roughness of polycrystalline silicon were analyzed with scanning electron microscope and atomic force microscope. After completion of thin-film transistors, I-V testing machine was used to test device characteristics. Both the optimum energy density (OED) and process window (OED margin) of excimer laser crystallization increased with the increase of amorphous silicon thin film thickness. When the film thickness was ≥47 nm, OED margin was 25 mJ/cm2. Provided with film thickness of 47 nm, grain size uniformity was 0.64 and at optimal level. When refractive index of amorphous silicon film was 4.5, the grain size uniformity of polycrystalline silicon was 0.45, which was much superior to that of polycrystalline silicon grain with refractive index of 4.38. For un-doped LTPS TFT(PMOS) made from amorphous silicon film with refractive index of 4.5, mobility ratio was 120.6 cm2/(V?s), threshold voltage ?1.4 V, and off-state current 53 pA; for that made from amorphous silicon film with refractive index of 4.38, mobility ratio was 112.4 cm2/(V?s), threshold voltage ?2.0 V, and off-state current 71 pA. Provided with amorphous silicon film thickness of 47 nm, OED margin of excimer laser crystallization and grain size uniformity of crystallized p-Si are both at superior level. In addition, improvement of refractive index of amorphous silicon film is also beneficial to enhancement of P-Si grain size uniformity and LTPS TFT transfer characteristics.