Due to the difference in charging and discharging of the full-bridge sub-module and the half-bridge sub-module, the capacitor voltage imbalance may occur when hybrid MMC operates at a reduced DC-link voltage. In this paper, third harmonic voltage injection based control method which can improve the above imbalance is proposed. First, the mechanism of the capacitor voltage imbalance problem is reviewed, and the boundary conditions when the capacitor voltage imbalance problem occurs is deduced. Further, considering of different DC-link voltage drops, the hybrid MMC operating area is calculated. Then, the effect of the third harmonic voltage injection on the capacitor voltage imbalance is analyzed, and the expansion effect on the operating area of ??the hybrid MMC is analyzed quantitatively. Finally, a double-ended simulation model with Matlab/Simulink is established, and the simulation results verify the effectiveness of the proposed method and corresponding analysis.