That large-scale wind power is connected to weak AC system or transmitted through HVDC transmission lines or series compensated AC transmission lines may induce Sub-Synchronous Oscillation (SSO) of power system, resulting in the disconnection of a large number of wind turbines and the damage of related equipment. To solve this problem, a simple and effective method is introduced to identify SSO by monitoring the oscillation waveform of the active power of the tie-line. The exponential function is used to fit the amplitude of the SSO, and the oscillation mode and the divergence rate of the SSO are determined. The corresponding relationship between the divergence rate of the oscillations and the number of wind turbines tripped is designed. Finally, a three-round wind farm tripping strategy under SSO situation is proposed. Theoretical analysis and simulation results based on PSCAD show that the proposed wind farm tripping strategy can effectively suppress SSO and ensure the safe and stable operation of the power grid. In addition, the wind turbines are preserved to the maximum extent under SSO situation and the wind power utilization capacity is enhanced. This work is supported by National Natural Science Foundation of China (No. 51537003) and Science and Technology Project of Southern China Power Grid Guangxi Power Grid Company (No. GXKJXM20170244).