Power losses on the upper and bottom insulated gate bipolar transistors (IGBTs) in half bridge sub-modules (HBSMs) are uneven when hybrid modular multilevel converters (MMCs) work as inverters. The loss of the bottom IGBT in HBSM is much higher than any other device, which results in the high thermal stress and failure rate. To address this problem, a device power loss distribution optimization method is proposed in this paper. Firstly, device power losses are calculated, and the distribution characteristic is analyzed. Secondly, the influence of sub-module capacitor voltage on the device power loss is analyzed, and it is found that the loss of the bottom IGBT can be reduced for a low sub-module capacitor voltage. Thirdly, through combining third harmonic voltage injection and the differential distribution of the arm output voltage reference, a loss distribution optimization method based on low sub-module capacitor voltage is proposed. Finally, the simulation and experiment results verify that the proposed method can improve the uneven device loss distribution of hybrid MMC and the operation reliability of the converter.