The invention discloses an MMC (Modular Multilevel Converter) upper and lower bridge arm separation control method, comprising the steps that a power grid current instruction respectively multiplies by current instruction coefficients ku and kd of upper and lower bridge arms to be used as current inner ring instruction values of the upper and lower bridge arms; respectively sampling current of the upper and lower bridge arms to be used as feedback signals; respectively carrying out closed-loop control on the upper and lower bridge arms through the decoupling control of a decoupling controller and signal processing so as to realize the current output of the upper and lower bridge arms. According to the MMC upper and lower bridge arm separation control method, a separation control method is adopted, an MMC is adaptive to the current balance control of the upper and lower bridge arms under the condition that difference exists in electric reactors of the upper and lower bridge arms; the MMC is adaptive to the asymmetric operation of the upper and lower bridge arms; different levels of current is output by the upper and lower bridge arms; under the condition of not increasing a circular current controller, the effective control on circular current can be realized; the method is used as a selectable novel control method of the three-phase MMC, so that the method has wide application prospects.