Static Synchronous Compensator（STATCOM）is considered an effective means to keep the voltage stable at the connection point between a wind power plant and the grid. The dc voltage imbalance of a cascaded STATCOM in a wind power generation system will affect the quality of output voltage waveform and the operation of the STATCOM. To solve the problem of the dc voltage imbalance, the usual approach is to add some special control algorithms to the controller, making the controller rather complex and bringing extra parameters that should be adjusted. This paper proposes to use a voltage feedforward space vector modulation (FFSVM) method in a cascaded STATCOM. This method can obtain perfect output waveform under the condition of dc voltage imbalance and make them converge. This paper extends its utilisation from a two-cell cascaded inverter into a STATCOM with three or above cells cascaded in one phase. Considering the fact that the dc voltage differences among the cells in a STATCOM are small, some extreme situations of the dc voltage distribution are excluded, and the vector selection is simplified. Thus the application method of FFSVM in a multi-cell cascaded STATCOM is developed. If the number of cells increases further, there will be more output vectors, but the method of vector selection is still applicable. A simulation model of a three-phase three-cell cascaded STATCOM is built in Matlab/Simulink. Simulation results verify that a cascaded STATCOM adopting the proposed modulation method can accurately output waveforms, and simultaneously suppresses dc voltage divergence by using output vectors conducive to convergence.