Abstract: With the implementation of the "dual-carbon" goals, the integration of wind farms and hydropower plants via AC/DC transmission systems has become a trend. However, research on the oscillation characteristics of direct-drive wind farms, hydropower plants, and AC/DC systems remains scarce. Firstly, for wind power and hydropower transmitted via AC/DC transmission systems, a modular modeling method is used to establish a full-order small-signal model of the system. Then, using the eigenvalue analysis method, we identify and analyze the different oscillation modes of the system before and after the integration of the direct-drive wind farm. Finally, by analyzing the changes in modal root trajectories, we examine the impact of different control parameters on the system’s oscillation characteristics. The research results indicate that when a direct-drive wind farm feeds power into the hydropower system via an AC/DC transmission system, its impact on both the ultra-low-frequency and low-frequency oscillations of the original system is relatively minor. In addition, the wind turbine will engage in the subsynchronous oscillation mode of the original system, while also generating new subsynchronous oscillation modes. In the case of improperly configured parameters, the system is exposed to the risk of subsynchronous oscillation. The findings of this study can serve as a theoretical foundation for practical engineering applications.