To explore the clustered voltage balancing mechanism of the cascaded H-bridge static synchronous compensator (STATCOM), this paper analyzes the causes of unbalanced clustered voltage. The negative-sequence current caused by the compensation of unbalanced reactive power or detection and control errors and the zero-sequence voltage caused by voltage drift of the STATCOM neutral point contribute to unbalanced clustered voltage. On this basis, this paper proposes a control strategy to inject negative-sequence current and zero-sequence voltage simultaneously. The injection of negative-sequence current may cause current asymmetry in the grid, and the zero-sequence injection has a relatively limited balancing ability in the clustered voltages. The proposed control strategy can not only generate a faster balancing response than the traditional zero-sequence voltage injection method, but also lower the extent of current asymmetry compared with the traditional negative-sequence current injection method. Then, the negative-sequence current and zero-sequence voltage injection are further transformed into the dq frame to establish a unified frame. The effectiveness of the proposed control strategy is verified by the simulation and experimental results.