To improve the rate performance and decelerate the voltage decay of Li-rich layered oxide cathode materials, a series of cathode materials Li1.2[Mn0.7Ni0.2Co0.1]0.8−x Se x O2 (x = 0, 0.07, 0.14 and 0.21) was synthesized via co-precipitation. Based on the characterization results, it can be concluded that uniform Se6+ doping can improve the degree of crystallinity of Li2MnO3, resulting in a better ordering of atoms in the transition metal layer of this type of cathode materials. In the electrochemical experiments, compared to un-doped samples, one of the Se doped samples (LLMO-Se0.14) exhibited a longer sloping region and shorter potential plateau in the initial charge curves, a larger first coulombic efficiency (ca. 77%), better rate capability (178 mAhm g−1 at 10 C) and higher mid-point voltage (MPV) retention (ca. 95%) after 100 cycles. These results prove that Se doping can effectively improve the rate capability and decelerate the voltage decay process of these cathode materials during cycling via suppressing the oxidation process of O2− to O2 and curbing a layered-to-spinel phase transformation. The above-mentioned functions of Se doping are probably due to the higher bonding energy of SeO than that of MnO.