Abstract: A permanent magnet synchronous generator (PMSG)-based wind farm can be used to realize the high-capacity and long-distance transmission of the wind power through a voltage source converter based HVDC (VSC-HVDC). They may face sub-synchronous oscillation (SSO) or high frequency oscillation (HFO) issue, which has been well studied recently. However, the medium frequency oscillation (MFO) whose frequency between SSO and HFO may also exist in a PMSG-based wind farm when connected to a VSC-HVDC, which is worthy of researches. In this paper, the mechanism and characteristics of MFO are explored based on the single input single output (SISO) equivalent sequence impedance model of the interconnected system. In addition, the influence of parameters from both PMSG and VSC-HVDC on the MFO characteristics is analyzed as well. And the analysis is validated by simulation. Last but not least, a virtual impedance based impedance reshaping control in the wind farm VSC (WFVSC) station is proposed. With the method, MFO of the interconnected system can be suppressed effectively, which is also validated by simulation, and thus provides important reference for practical engineering.