Voltage unbalance in clusters DC link exists during experiments， and the unbalance degree increases with increase of the compensation current. DC link voltage balance among clusters is important for the safe operation and performance of cascaded H-bridge multilevel STATCOM. However， there is lack of reason analysis about voltage unbalance in clusters DC link nowadays. Through decomposing the positive and negative sequence of system voltage， interaction between the positive sequence voltage and the negative sequence current in cascaded H-bridge topologic structure， which has influence on active power， is analyzed. It also explained the physical reasons for voltage unbalance in clusters DC link， and a quantitative analysis on the cluster unbalance is given. Simulation and experimental results proved the validity of this method， and provided the reasonable basis for solution of voltage unbalance in clusters DC link.