Abstract: Series-connected DC wind farm is one of the promising solutions for the collection and transmission of large-scale wind energy in offshore regions. However, in this scheme, the wind turbines are coupled with each other, which may cause the switch of control modes. First, the components of the wind farm are classified according to their control modes, and corresponding impedance models for each part are established, leading to an equivalent impedance model for the entire system. Based on this impedance model and operational characteristics of the series-connected DC wind farm, a stability criterion is proposed. Compared with conventional criteria, the proposed criterion considers the coexistence of multiple control modes in series-connected DC wind farms, and combines the impedances of each part based on its control mode, which can accurately and effectively assess the stability of the system. Furthermore, an active damping method is proposed to optimize the impedance of DC wind turbines to improve the stability of the system. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed active damping method are verified through simulation experiments.