Abstract: Safe and stable operation of modular multilevel converters（MMCs） used in high voltage direct current systems（HVDC） is concerned. The circulating current and DC power fluctuation need to be controlled respectively under unbalanced grid conditions. Firstly, the DC power fluctuation of the whole bridge was analyzed by instantaneous power equations under unbalanced voltage conditions. The capacitor fluctuation and circulating current components were analyzed by the modulation function and submodule port voltage equations. Due to the double line frequency components of positive, negative and zero sequences flowing the whole bridge, thus, a circulating current suppression strategy of individual phase control based on theoretical analysis was proposed. The separate control of positive and negative sequence components and the coordinate transformation were avoided by this method. It can not only suppress the circulating current, but also eliminate the DC current ripples. Finally, the system characteristics under unbalanced grids was analyzed and verified by the Fourier analysis, the control strategy was tested by utilizing the Matlab simulation and experiment platform.