Abstract: The active frequency support control is generally adopted in the renewable energy to adapt to the large-scale grid connection of renewable energy. However, the active frequency support control may induce the subsynchronous oscillation (SSO) in the wind-thermal coupling system. To address that, a model of the wind-thermal coupling system with active frequency support control is decoupled based on the effective open-loop process theory and equivalent to a single-input single-output system. Then, an improved stability criterion is constructed for SSO analysis. Compared with the original criterion, the proposed criterion is able to judge whether the SSO occurs in the coupling system through the phase characteristics of the Bode diagram based on the characteristic transfer function. According to the judgment results of the proposed criterion and based on the phase compensation principle, a phase reshaping controller is proposed to increase the stability margin of the coupling system within the subsynchronous frequency range. Finally, the validity of the proposed criterion is verified by the time-domain simulation. The time simulation and the RT-LAB simulation results show that the proposed SSO suppression strategy achieves a satisfactory frequency adjustment effect under the premise of ensuring the stability of the wind-thermal coupling system.