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 are carried out, the simulation results show that the proposed unified active damper can enhance the stability of the system.