Subsynchronous voltage modulation strategy for voltage source inverter (VSI) in parallel with system is presented, which is exclusively for subsynchronous oscillation (SSO) mitigation use. Approximate electrical damping provided by the strategy for suppressing subsynchronous resonance (SSR) induced by fixed series capacitor (FSC) is formulated by complex torque coefficient approach. The analysis result demonstrates that when the phase difference between system voltage of access point and subsynchronous voltage output of VSI satisfies some condition, VSI can provide positive electrical damping. And the related elements which influence the size of this positive electrical damping are analyzed. The controller of VSI is designed by dq transformation, which can mitigate SSO and maintain voltage on DC side of inverter by decoupled control. The simulation result with the methods of test signal and time domain simulation reveals that the subsynchronous voltage modulation strategy based on VSI in parallel can damp SSO effectively.