The invention discloses an additional active signal-based balance control system and method for a VSC-MTDC system. When a disturbance or a fault occurs in the VSC-MTDC system, the method comprises the following steps: (1) detecting a DC voltage output by a DC side of a constant DC voltage converter station in real time, and calculating a voltage difference value between the detected DC voltage and the maximum DC voltage value output by the DC side of the constant DC voltage converter station; (2) determining the ratio of the voltage difference to a voltage drop coefficient in a rectification mode of the constant DC voltage converter station to obtain an additional value of an active power; and (3) superposing the obtained additional value of the active power into an active power set value of a constant active power converter station controller, compensating the unbalanced power of the VSC-MTDC system through additional values of active powers which are evenly assumed by convertor stations in the VSC-MTDC system in the absence of inter-station communication to maintain active power balance and DC voltage stabilization of a constant DC voltage station.