Regarding to the lowfrequency oscillation problem brought by AC/DC hybrid system involving the wind power, the characteristics of weak or negative damp lowfrequency oscillation for AC/DC hybrid system with permanent magnetic synchronous generator (PSMG)based wind farms are studied in this paper. Firstly, a DC transmission dynamic model of is analyzed by considering direct drive wind turbines, full power ACDC converters, as well as the constant current at the rectified side and constant voltage at the inverter side. Then, a differential equation model is established to study the stability of small interference in PSMGbased wind farms. The stability of small interference of a DC transmission system with CEPRI 36 nodes is also analyzed under four different connection modes. In addition, the effects of two operation modes of wind farms participating active and reactive power dispatching on the characteristics of weak or negative damp lowfrequency oscillation are analyzed. The results show that the weak or negative damp lowfrequency oscillation of the system is better when the DC transmission system works under the mode of bipolar neutral grounding at both ends.