DC capacitor voltage balance is the prerequisite for the safe and stable operation of static synchronous compensator (STATCOM). After analyzing the phenomenon of imbalance of DC capacitor voltage of cascaded static synchronous compensator, a method based on active power vector addition is presented to solve this problem. Through adding active power voltage vector and hierarchical control (the upper and the lower control), control signals can be adjusted and active power can be distributed on demands to supply the loss difference among the cascaded modules. The upper control realizes the whole active and reactive power control by decoupling, the lower control realizes voltage balancing control by adding active power voltage vector. The tactic is clear of physics meaning, simple of algorithm and few coupling among control signals. Simulation results based on Matlab/Simulink and experimental results verify that the presented tactic is correct, effective and workable.