Abstract: The risk of sub-synchronous resonances may exist in the power electronic transmission systems composed of PMSG based wind plant farm and flexible HVDC system. To suppress the resonances, small-signal modeling method was used to establish the integrated mathematical model considering the internal dynamic of MMC and the dynamic process of PLL. Based on the root locus, the effects of active power, controller parameters and equivalent resistance on the stability of the system were analyzed, and the dominant participating variables and factors of sub-synchronous resonance modes were also given. The results show that the characteristics of sub-synchronous resonance are mainly related to the q-axis parameters of MMC and wind farm plant. The conclusion that stability of the system can be improved by properly increasing the line resistance was also achieved. On the basis of this conclusion, the damping control strategy adding to MMC outer loop controller was proposed. The influence of damping controller parameters on system stability is studied, and the stability region of parameters is given. The correctness of the modeling and damping control strategy in this paper is verified by electromagnetic transient simulation models.