A circulating current suppressing controller（CCSC） is designed to eliminate the inner balancing currents in modular multilevel converter（MMC）. The CCSC uses the inner mathematical model of MMC through a double fundamental frequency negative sequence rotational frame to decompose the three-phase alternative circulating currents to two DC components. The designed CCSC can minimize the distortion degree of the arm current significantly such that the arm current can closely approximate the fundamental sine wave. Simulation results based on PSCAD/EMTDC show that the CCSC can suppress the inner circulating current of MMC efficiently without increasing the arm reactance. It is also shown that this additional controller doesn't affect the AC side dynamics of MMC.