DC-link capacitor voltage balance is an important condition for safe and reliable operation of Cascade STATCOM. A novel DC-link capacitor voltage control strategy is proposed for the Cascade STATCOM based on step wave modulation. The purpose of capacitor voltage balance can be achieved by changing the step wave superposition strategy of each H-bridge. Compared with other control strategy, the proposed control strategy does not need adding additional hardware devices, effectively reduce device costs. The software algorithm is very simple without changing the conduction angle, modulation ratio and other parameters, and the quality of output current can't be affected. The simulation model based on Matlab shows the feasibility and effectiveness of the proposed method.