As one of the promising candidate cathode materials for the high-performance lithium-ion batteries, Li-rich layered oxides still suffer from a series of critical drawbacks, such as voltage decay, oxygen release and irreversible migration of transition metal ions, etc. In this work, Li-deficient method has been confirmed as an effective approach to improve the overall electrochemical performances of Li-rich cathode. The optimized lithium-deficient Li-rich layered cathode exhibits splendid discharge capacity of ∼297 mAh/g at 0.1C and prominent rate performance of ∼143 mAh/g at 5C. Subsequently, neutron diffraction in combination with Raman spectroscopy are applied to explore and clarify the underlying mechanism for improved performances. It was found that the Lithium-deficient induced nickel migration and oxygen vacancy play an significant role in improving electrochemical performances, because migration of nickel into Li layer is able to expand the Li layer spacing and reduce the Li/Ni antisite, leading to facilitated diffusion of lithium ions. Moreover, the formation of oxygen vacancy is able to promote anionic redox processes and suppress the gas release, thus leading to higher capacity. The results present valuable structural insights into the influence of lithium deficiency and provide guidance for the development of Li-rich cathode materials.