Carbon nanotube (CNT) fibers spun from vertically aligned CNT arrays hold great promise in promoting CNT's practical applications. Their production and properties strongly depend on the spinnability of the arrays. Herein, we study the influence of Al2O3 buffer layer on catalyst morphology and the spinnability of CNT arrays. Long and vertically aligned CNT arrays have been obtained from a wide range of Al2O3 buffer layer thickness, but the spinnable ones have only derived from a narrow range of the thickness. It is further found that the Al2O3 buffer layer can regulate the size and size distribution of the catalyst particles through balancing surface diffusion and inter-layer diffusion. Small, dense, and uniform-distributed nanoparticles are fingerprinted as the optimal catalyst for growing long and spinnable CNT arrays and can be obtained at a proper thickness of buffer layer. By using a tailored tri-layered Fe/Al2O3/SiO2 catalyst, the obtained CNT arrays could reach a height of 500-800 mu m and are highly spinnable.