The invention provides a control device for a flexible direct current (DC) transmission system of a modular multilevel converter (MMC). The control device comprises a home station and an opposite station which are arranged symmetrically, wherein, the home station is connected with the opposite station through a lead; and the control device of the home station and the opposite station comprises a converter station-class control layer, a converter-class control layer and a converter valve-class control layer which are hierarchically designed. The control device for the flexible DC transmission system of the MMC has the beneficial effects that the operating reliability of the system is ensured through the converter station-class control layer and the converter-class control layer; and by controlling the converter station-class control layer, the converter-class control layer and the converter valve-class control layer, independent control on active power and reactive power is realized, circular current among an upper bridge arm, a lower bridge arm and phases of a current converter is inhibited, and capacitive voltage difference among submodules is reduced so as to ensure that the capacitive voltage of the submodules of the current converter is consistent.