There are serious problems tending to result in resonance and instability because of the mutual coupling of multi-inverters in the system through the grid impedance. The traditional active damper can suppress the system resonance by impedance adaptation with virtual impedance. However, it cannot fully decouple multi-inverters system and the grid impedance. What is more, voltage source harmonics such as grid background noise cannot be effectively suppressed. To address them, this paper proposes a unified active damper that uses a combination of a parallel structure and a serial structure; it can reduce the influence of the grid impedance and suppress system's resonance and harmonic amplification at the PCC. In order to verify the effectiveness of the proposed scheme, a Simulink of the multi-inverters system setup is carried out; the simulation results show that the proposed unified active damper can enhance the stability of the system.