There is active power coupling between ac system and dc system in ac/dc interconnected power grid, and modular multilevel converter based HVDC (MMC-HVDC) system could not completely isolate the fault from ac system to dc system or dc system to ac system. In order to achieve the effect of independent decoupling operation of HVDC system and make full use of "firewall" function of the MMC-HVDC system, the battery energy storage system (BESS) based MMC-HVDC system is a preferred choice. For the BESS based MMC-HVDC system, this paper firstly analyzes the basic structure and operation principal. Then, based on numerical integration and nested fast and simultaneous simulation approach, this paper derives the Thévenin equivalent circuit of BESS based SMs and BESS based MMC. The case studies are performed on a ± 500 kV 2-ternimal MMC-HVDC system. The time-domain simulation results show that the ac side active power and the dc side power of the BESS based MMC-HVDC system can be decoupled from each other, and the transmitting power during and after the ac or dc fault can be kept.