In order to solve the problem of power quality deterioration of modular multilevel converter?based battery energy storage system (MMC?BESS) under the disturbance of background harmonic and dead time effect, a harmonic suppression strategy of MMC?BESS based on arm current control is proposed in this paper. To investigate the harmonic disturbance of the MMC?BESS in the medium voltage application scenario, firstly, the influence of AC background harmonic and dead time effect on the MMC?BESS is analyzed, and the mathematical model is established. Secondly, the decoupling models of AC current, DC arm circulating current and AC circulating current of the MMC?BESS are also established. On this basis, the harmonic suppression strategy of MMC?BESS based on arm current control is proposed, and the detailed controller parameter design process and performance analysis are given. Finally, the effectiveness and correction of the proposed harmonic suppression strategy are verified by simulation and experiment.