Due to the high discharge capacity, lithium-rich layered oxides are regarded as one of the most promising cathode materials for lithium-ion batteries (LIBs). In the present circumstances, however, they have been suffering from rapid voltage fading and poor rate capability. Herein, a lithium-rich layered oxide of Li1.2Mn0.6Ni0.2O2 (PLMNO) with a specific fine microstructure is successfully prepared through a solvothermal method followed by a stepwise calcination process. The PLMNO material consists of primary nanoparticles assembled into a 3D porous rod-like hierarchical micro/nanostructure, which exhibits a facile pathway for the transportation of lithium ions and electrons. The initial charge and discharge capacities at a current density of 0.1C (1C = 200 mA g−1) are 348.3 and 263.4 mAh g−1, respectively, with a initial coulombic efficiency of 75.6% in the voltage range of 2.0–4.8 V. At a large rate of 1C, a high discharge capacity of 197.3 mAh g−1 can be achieved with capacity retention of 88.5% after 200 charge/discharge cycles. In addition, the voltage fading during charge/discharge cycles is remarkably suppressed. The superior electrochemical properties make this layered Li-rich material of PLMNO a very promising cathode material in LIBs.