Multiple occurrences of sub-synchronous oscillations have been observed following the integration of a wind farm into a four-terminal ring-structured modular multilevel converter (MMC)-high-voltage direct current (HVDC) grid in China. To address repeated oscillation, this paper develops a small-signal model of a direct-drive wind farm integrated into the four-terminal MMC-HVDC system, and reveals the underlying mechanism of the sub-synchronous oscillations observed in the actual project. It is found that a direct-drive wind farm is closely coupled only with the directly connected MMC, while other MMCs have little impact on sub-synchronous oscillation modes. Therefore, when studying the sub-synchronous oscillation issues involving the interaction between the wind farm and the flexible DC system, a simplified model can be established using a single-ended flexible DC system directly connected to the wind farm, ignoring the influence of other MMCs. It is also found that in the system where a direct-drive wind farm interacts with a directly connected MMC, both active power-coupled sub-synchronous oscillation modes and reactive power-coupled sub-synchronous oscillation modes coexist. As the output power of the wind farm gradually increases, the damping of the active power-coupled sub-synchronous mode increases, while the damping of the reactive power-coupled sub-synchronous mode decreases, reproducing the onsite phenomenon where an increase in wind farm active power aggravates sub-synchronous oscillations. The smaller the integral gain of the outer loop of the wind farm’s reactive power controller and the proportional gain of the phase-locked loop (PLL), and the larger the integral gain of the q-axis component of the AC voltage outer loop at the wind farm-side converter station, the more stable the reactive power-coupled oscillation mode becomes. By optimizing these parameters, the risk of system instability can be reduced. A simulation model of a direct-drive wind farm connected to a four-terminal flexible DC grid was built using the electromagnetic transient simulation software PSCAD (power systems computer aided design), and the correctness of the above theoretical analysis was verified.