The layered 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 lithium-rich manganese-based solid solution cathode material has been synthesized by two-step co-precipitation method. X-ray diffraction studies reveal that the as-prepared material has a typical layered structure with R-3m and C2/m space group. The surface morphology of the [Mn4/6Ni1/6Co1/6]CO3 precursor and cathode material 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 have been examined using scanning electron microscopy. Electrochemical performances, including discharge capacity, cycling performance and rate capability also have been investigated. The results show that the cathode material 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 prepared by two-step co-precipitation method has a good electrochemical performance, which can deliver a high initial discharge capacity of 276.70 mAh g−1 at the current density of 25 mA g−1. When cycling at 125 mA g−1, the material indicates an initial discharge capacity of 211.31 mAh g−1 and 196.64 mAh g−1 after 50 cycles, capacity retention is 93.06% and its coulombic efficiencies is greater than 98% at each charge/discharge cycling. Furthermore, the as-prepared material 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 also shows an excellent rate capacity.