Abstract: The wind-power generation is a kind of renewable energy technology with the good development prospects at present. As the problem of wind farm resonance becomes more and more serious. Moreover, because of the negative damping of the VSC(voltage source converter) output impedance in partial frequency domain, sometimes there is a problem of harmonic instability in wind farms. The purpose of this paper is to analyze the mechanism of harmonic instability in wind farms. Firstly, the harmonic model of wind farms is built in this paper. The VSC impedance model base on the control parameters is established to explore the output characteristics of the VSC in the wind turbine. The transmission and amplification rules of harmonics in the VSC control loop are explored. Further, the influence factors of harmonic instability are analyzed by using Nyquist stability criterion. Meanwhile, passive damping is used to suppress harmonic instability.