Abstract: The modular multilevel converter with partly integrated battery energy storage system（MMC-PBESS） can save construction costs while connecting energy storage, but its control is more complicated. For the MMC-PBESS topology where the lower bridge arm is connected to battery energy storage, a mathematical model and an equivalent circuit are established. On this basis, a capacitor voltage balancing strategy is given, a control strategy for upper/lower bridge arm separate control is proposed, and the operation boundary is analyzed. A simulation model is built in MATLAB/Simulink platform to simulate the operation conditions of different AC to DC power ratios. The proposed control strategy can achieve the battery charging function while maintaining the capacitor voltage balance. This strategy does not require additional circulation calculations, and the upper and lower bridge arm control is decoupled, making it simple and flexible.