With the development of new energy technology, large-scale access of power devices such as inverters to the grid will cause instability of the grid-connected system and harmonic amplification at the point of common coupling(PCC). In order to improve the stability of the grid-connected system and suppress the voltage harmonics, this paper first establishes a mechanism model for the harmonic source and builds a one-port network impedance model of the grid-connected system. For the convenience of analysis, the impedance model is simplified as an equivalent circuit of the RLC in resonant frequency range. Then, aiming at the problem of harmonic suppression, the voltage harmonic amplification coefficient method is proposed to quantitatively analyze the phenomenon of voltage harmonic amplification, and different types of active dampers are added to the equivalent RLC circuit. By analyzing the relationship between different types of active dampers and voltage harmonic amplification coefficient and harmonic suppression effect, the active damper with the best harmonic suppression effect is obtained to adjust the damping characteristics of the whole new energy power generation system and improve the stability of the grid-connected system. Finally, the feasibility of the method is verified by simulation.