Abstract: The proposal of "carbon neutral" and "carbon peak" goals, along with the implementation of policies such as low-carbon energy transformation, lead to an remarkable increase in the integration of new energy sources, thereby transforming the power system into a electronic one. The high power electronic power system is susceptible to frequency coupling, harmonic resonance and other issues that may result in system instability when severe. In view of this, in this paper the first step involves constructing a small signal model of wind turbine grid-connected system in closed-loop mode, based on the harmonic state space theory. Then, the harmonic state space matrix of permanent magnet synchronous wind power generation system was derived, followed by a successful establishment and qualitative analysis on the system frequency coupling relationship. The accuracy of the model is ultimately validated through a comparison between the simulation model results and the mathematical model results.