Abstract: Sub-synchronous oscillation (SSO) is observed in doubly-fed wind farms connected to series-compensated transmission system. In this paper, a positive-sequence and negative-sequence impedance model of doubly-fed induction generator (DFIG) was presented based on three-phase stationary coordinate system, considering grid-side converters (GSC), rotor-side converters (RSC) and phase locked loop (PLL). It was demonstrated that the dynamics of SSO was much faster than the outer control loops from the perspective of bandwidth. The impedance model was verified and compared respectively by theoretical analysis and simulative results. Then the influence of the impedance of GSC on the total impedance of DFIG was studied. Finally, the influence of factors such as power output, current loop controller parameters, system series compensation and numbers of generators on system stability was analyzed qualitatively based on Nyquist criterion, and parameter adjustment measures for Suppressing SSO were proposed. The SSO risk can be restrained by adjusting the generators output, reducing the number of generators, reducing the line series compensation, adjusting the RSC current loop parameters and other measures.