A control strategy of static synchronous compensator（STATCOM）based on supplementary voltage control for damping subsynchronous resonance（SSR）is proposed with detailed mathematical analysis on its mechanism.The proposed control scheme is then tested on a typical series-compensated transmission system through simulations.The results demonstrate its effectiveness in damping multimodal SSR.The comparison between STATCOM and static var compensator（SVC）indicates that at the same capacity，STATCOM outperforms SVC in damping SSR.The results are helpful to solving the SSR problems which exist in the series compensated transmission systems in China and abroad.The conclusions also provide valuable references for better applications of the STATCOM.