Abstract: It is a typical power electronics-based interconnected system which may have the risk of sub-synchronous oscillation when flexible high voltage direct current (HVDC) station connects to the permanent magnet synchronous motor (PMSM) based wind farm. To explore the key factors affecting the sub-synchronous oscillation, the linearized models of modular multilevel converter (MMC) considering the inner dynamics and grid-side voltage source converter (GSVSC) considering the dynamics of phase-locked loop (PLL) were established based on the state space modeling method. The influence of active power and PLL's parameters of wind farm on the admittance matrix of GSVSC was analyzed as well as the influence of active power, parameters of constant AC voltage controller and circulating current suppression controller on the output impedance matrix of MMC. The key factors affecting the stability were studied based on the generalized Nyquist stability criterion. A suppression control strategy simulating the performance of paralleled impedance at the points of common coupling (PCC) was proposed. Then, the parameter selecting range of the controller was analytically calculated. Finally, the effectiveness of the proposed strategy and the correctness of the calculated parameters were verified by the electromagnetic transient simulation models.