Hybrid Modular Multilevel Converter (MMC) has the ability to operate under reduced DC voltage and ride through DC faults without blocking IGBTs. Sub-module balancing constraint places the dominant limit on the feasible power transferring region compared to other constraints when it operates under reduced voltage. This paper analyzes the characteristics of the arm current and arm output voltage that meet sub-module balancing constraint, and calculates the corresponding maximum power operating region when hybrid modular multilevel converter operates under reduced voltage. A fast calculation method is proposed to determine the feasible operating region. The effectiveness and accuracy of the proposed method is validated by comparing calculation results with a scanning method and cross-verified by electromagnetic simulation. The impact of the full-bridge sub-module ration on the operating region is analyzed. A new design model regarding full-bridge sub-module minimum ratio is provided from the perspective of power transferring ability. This work is supported by Smart Grid Joint Fund of National Natural Science Foundation of China (No. U1766211).