To reduce the large first-cycle irreversible capacity loss of the Li-rich layered cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2, MoO3 has been introduced by a simple high-energy ball milling process. The electrochemical properties of cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 and the influences of different MoO3 amount on its electrochemical properties are discussed in detail. The first charge–discharge dQ/dV curves suggest that the MoO3 component provides additional sites for lithium ion insertion to compensate the lost Li sites caused by the simultaneous removal of Li+ and O2− during the activation of Li[Li0.2Mn0.54Ni0.13Co0.13]O2. With increasing MoO3 content from 0 wt.% to 20 wt.%, the first-cycle irreversible capacity loss of the composite decreases from 81.8 mAh g−1 to 1.2 mAh g−1. The composite with 5 wt.% MoO3 exhibits a good cycling stability with the discharge capacity of 242.5 mAh g−1 after 50 cycles, and the thickness of the MoO3 coating layer on the surface of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 is about 3–4 nm. However, with the increase of the addition content of MoO3, the cycling stability of the Li[Li0.2Mn0.54Ni0.13Co0.13]O2–MoO3 composite is decreased.