The invention discloses a control and switch method for redundant submodules of a modular multilevel converter. The converter monitors a voltage value of a redundant submodule support capacitor in the normal running process. When voltage of two ends of the redundant submodule support capacitor of a phase of bridge arm is larger than or equal to a first preset value and smaller than or equal to a second preset value, the redundant submodules are in a switch-out state; when the voltage of two ends of the redundant submodule support capacitor is smaller than the first preset value, the on-off states of a first switch tube and a second switch tube are determined according to a modulation mode and a submodule direct voltage balance control strategy; and till the voltage of two ends of the redundant submodule support capacitor is equal to the second preset value, the second switch tube is connected for afterflow, otherwise, a message of a overvoltage fault of the redundant submodules is sent, so that the protection action is executed. According to the control and switch method, the redundant submodules can be connected to bridge arms at the highest speed to replace fault submodules, so that the stability of voltage of the direct current side of the converter is maintained, the balance in voltage between capacitors of the submodules of the upper bridge arm and the lower bridge arm is kept, the reliability is improved, and the availability ratio is increased.