The Li-rich manganese-based cathode of Li1.2+xNi0.1Co0.2Mn0.5O2(x=0, 0.036, 0.060, 0.096) was synthesized by a rheological phase method. The effect of extra lithium amount on the structure and electrochemical properties was discussed. X-ray diffraction (XRD) indicates that all samples had typical α-NaFeO2 structure and small cationic mixing degree. Scanning electron microscope (SEM) showed that the materials with different amount of extra lithium were relatively uniform, smootn surface. The result of electrochemical tests showed that the material with optimal amount of extra lithium (x=0.036) achieved the best electrochemical performance. The tesing of electrical performance between 2~ 4.8V at 0.05C showed that the initial discharge capacity was up to 215.3mAh?g-1 at 25℃ with the initial efficiency of 69.7% and 297.1mAh?g-1 at 55℃ with the initial efficiency of 84.6 %。After 50 cycles at 0.2C, the capacity retention was 89.0% at 25℃ and 87.8% at 55℃. And when x=0.036, the material had the best rate performance.