Lithium-rich cathode materials have attracted great interests due to their advantages, such as high energy density and wide voltage window. However, their shortcomings like low initial coulombic efficiency and poor cycle performance have hindered their commercial application. Lithium-rich cathode materials Li_1.2Ni_0.13Co_0.13Mn_0.54O_2-xCl_x (x=0, 0.025, 0.050, 0.100) with different molar ratios of chloride ion (Cl^-) doping were prepared by a co-precipitation method. The regulation mechanism of Cl^- doping on the improvement of the electrochemical performance was investigated by X-ray photoelectron spectroscopy, in-situ X-ray diffraction and galvanostatic intermittent titration analysis. Compared with the pristine material, the initial coulombic efficiency of the cathode material with Cl-doping in a molar ratio of 0.05 increases from 72.8% to 81.5% at 0.2 C, and the capacity retention increases from 57.9% to 79.1% after 200 cycles at 1 C. It is indicated that Cl-doping can regulate the electrochemical behavior of O^2- by oxidizing the more into O^n- (n<2) and less production of oxygen gas, thus reducing the structural deterioration. Meanwhile, the larger ion radius of Cl^- can expand the layer spacing, reduce the polarization and accelerate the lithium ions diffusion in the lithium-rich and manganese-based cathode material, thereby enhancing the initial coulombic efficiency and the cycle performance.