A dense LiCoO2 microcrystalline buffer layer for an all-solid-state lithium battery is designed and prepared as a stable interface using a magnetron sputtering method. The LiCoO2 microcrystalline buffer layer possesses a large interfacial specific surface area and excellent interfacial stability. The interfacial contact resistance of the modified interface is reduced by 1279 Ω compared to the unmodified interface. The mode of the structure of the modified interface is established by material studio, and the potential energy barrier is calculated by ab initio theory based on the structure model. The results show that the barrier to the migration of interfacial Li ion is only 0.097 eV, which is only 1/5 of that in the grain interior. The electrochemical performance is measured by using a cathode with a slurry mixture composed of LiNi0.5Co0.2Mn0.3O2 and polymer electrolyte. The results show that the electrochemical properties of the all-solid-state lithium batteries are significantly improved by the addition of the LiCoO2 microcrystalline layer, with the battery showing an initial discharge capacity of 149.5 mAh g−1 and a discharge capacity of 121.2 mAh g−1 after 50 cycles at 0.1 C and 60 °C.