Modular multilevel converters (MMCs) have the problem of three-phase internal power imbalance, which easily leads to the damage of system stability and the increase of system loss. To address internal power imbalance of MMCs, this paper analyzes this problem and proposes a control method based on the injection of positive and negative sequence circulating current. The proposed method mainly includes four parts: the extraction of positive and negative sequence circulating current; the calculation of circulating current command value; the calculation of the number of input sub-modules; the acquisition of trigger signal. By injecting the extracted positive and negative sequence circulating current, this study realizes the balanced distribution of power between sub-modules of MMC and reduces switching losses. The simulation results show that the proposed method can significantly reduce the internal power imbalance of the MMC system, improve system stability, and reduce system losses.