Transonic buffet can induce shock wave oscillations and flow separation, leading to lift fluctuations and increased structural dynamic responses, which necessitates load alleviation and response control. Based on small-amplitude skin vibration, this paper designs closed-loop active control laws with time delays to reduce lift fluctuations and structural responses in the plunge and pitch degrees of freedom of an airfoil under fluid-structure interaction. The influence of different control parameters is analyzed from the perspective of flow field evolution, with a focus on comparing the transonic buffet responses and control effectiveness between the fluid-structure interaction mode and the forced vibration response mode. The results indicate that under the forced vibration mode, the optimal control performance is achieved with a control law delay time of 0.25 cycles and the skin vibration center located at 0.55 chord length. In this configuration, the suppression of flow separation is most effective, thereby mitigating shock wave motion and achieving a buffet load reduction rate of 94.5%. Under fluid-structure interaction, the transonic buffet load reduction rate is 91.8%. Compared to the forced vibration mode, the buffet load before control is larger, the buffet response frequency changes, and flow separation becomes more pronounced.