Abstract: The start-up and operation processes of modular multilevel converter（MMC） are divided into three stages: uncontrollable precharge, controllable precharge and normal operation. The balance of sub-module capacitor voltages in the three stages is premise and key for the safe and stable operation of MMC. A novel capacitor voltages balance method, which is applicable to the three stages, was proposed for the MMC using the carrier phase shift modulation. The dual voltage-sharing coefficients balance method was proposed according to the capacitor voltage imbalance mechanism. Voltage-sharing coefficients k1 i and k2 i were obtained related to the capacitor voltage of each sub-module and the average voltage value of the single bridge arm. The capacitor voltage balancing of the uncontrollable precharge stage was achieved by slightly adjusting the duty cycle of the MOSFET in the self-power supply of the sub-module according to k1 i. And the balance of the controllable and normal operation precharge stage was achieved by slightly turning the duty cycle of the IGBTs in the sub-module according to k2 i. Finally, the feasibility and effectiveness of the proposed method is verified by a single-phase 3 level MMC prototype.