Abstract: In order to reveal the mechanism of near fundamental-frequency (NFF) oscillation in the doubly-fed induction generator (DFIG) based wind farms connected with modular multilevel converter based high-voltage DC (MMC-HVDC) transmission system, we proposed an effective NFF oscillation suppression strategy, and established the detailed impedance models of DFIG-based wind turbine generator (WTG) and wind farm side MMC considering negative-sequence control loops based on harmonic state-space method, which were validated by frequency-scanning. Then, the mechanism and the key influencing factors of the NFF positive- and negative-sequence oscillations of the MMC-HVDC connected DFIG-based wind farm were revealed by using the impedance analysis method. The research results show that the phase-locked loop (PLL) and current inner loop of DFIG-WTG are the key influencing factors of the NFF oscillation. On this basis, an NFF oscillation suppression method for wind farm connected with MMC-HVDC transmission system was proposed. The method adds a damping loop to the PLL of WTG to realize NFF positive-sequence oscillation suppression. Finally, the NFF oscillation phenomena in the practical system were reproduced and the effectiveness of the proposed oscillation suppression method was verified by time-domain simulations.