Abstract: Modular multilevel converter-based battery energy storage system(MMC-BESS) has uneven capacitance voltages between phases, which can easily lead to circulating current problems. The traditional control methods have complicated parameterization, insufficient dynamic response, and poor control performance under dynamic conditions, which makes it difficult to achieve good circulating current suppression effect for MMC-BESS system. At the same time, MMC-BESS is a nonlinear system, and the traditional control methods cannot control it precisely. To address the above problems, a circulating current suppression strategy based on passive second-order super-twisting control is proposed. Firstly, a passive control based on the Euler-Lagrange(E-L) system model is established to correct the system performance by adjusting the energy function to realize the global stability of the system. Then, the super-twisting second-order sliding mode control is added to improve the passive controller, which eliminates the external perturbation of the system while realizing the fast tracking of the reference value to achieve the fast suppression of the circulating second octave component of the loop current. Finally, the simulation is carried out on the Matlab/Simulink platform and compared with the PI control, passive control and sliding mode control. The simulation results verify the superiority of the proposed passive super-twisting second-order sliding mode control strategy.