Abstract: Subsynchronous oscillation problems exist in wind power gathering areas as northwest China.The mechanism is not clear about synchronous oscillation induced by large capacity direct drive permanent magnet synchronous generator and dynamic reactive power compensation devices connected to weak network. In this paper, linear analysis model was deduced involving converter of direct drive permanent magnet synchronous generator（D-PMSG） and static var generator（SVG）, thus input impedance model and transfer function of whole system were obtained. Methods were discussed for judging stability of two-input two-output（TITO） system in synchronous coordinate system. By means of impedance analysis, transfer function pole analysis and time domain simulation, coupling mechanism and influencing factors of subsynchronous oscillation were proposed.