Abstract: In view of the subsynchronous oscillation（SSO）caused by direct-drive wind farms via voltage source converter based high voltage direct current（VSC-HVDC）transmission system,existing research mainly focuses on the analysis of wind turbine operating in the maximum power traking region. In addition to the maximum power tracking region,the direct-drive permanent magnet synchronous generator（D-PMSG）has power limiting region and high wind speed region. Firstly,a small-signal model of the grid-connected system is established. Then,all operation regions of D-PMSG are analyzed based on the operation characteristic curve of D-PMSG,and the SSO modes in the system are analyzed by eigenvalue method under all operation regions. The theoretical analysis is verified by the PSCAD/EMTDC simulation. Finally,the mechanism that the pitch angle control affects on the stability of the system is analyzed. The results show that the higher wind speed and the higher output power can lead to the better stability under all operation regions. The output power has a great impact on the damping ratio of SSO at low wind speed and has less influence on the damping ratio of SSO at high wind speed.The pitch angle control does not directly participate in system SSO interaction,but affects the stability of the system by changing the power flow. In practical operation,if the power grid has no restriction on the output power of the wind farms,the maximum output power of the D-PMSG should be maintained as far as possible to improve the stability of the system;If the power grid restricts the output power of wind farms,the output level of a single wind turbine generator should be improved by tripping measures.