Large scale wind turbine connected to the power grid is easy to induce subsynchronous oscillation. “Curse of dimensionality” will be encountered when subsynchronous oscillation mode is analysed and evaluated based on the full scale model of wind farm. It is urgent to study the fast and effective analysis method of wind farm interconnected subsynchronous oscillation. It is urgent to research the fast and effective analysis method of subsynchronous oscillation when wind farm is connected to power grid. Therefore, a reduced order calculation method of subsynchronous oscillation mode of PMSG based wind farm connected with power grid based on Arnoldi method is proposed..This method can quickly obtain the eigenvalues corresponding to the subsynchronous oscillation mode through iterative approximation when PMSG based wind farm connected to the power system. This method can be used to analyze subsynchronous oscillation of large-scale wind farm connected to the power grid, and provide an effective means for the evaluation of it. The effectiveness, applicability and rapidity of the proposed method are verified under stable and unstable states, different collector network structures and different wind farm scales. The results present that the proposed method can quickly and accurately obtain the subsynchronous oscillation mode in the system.