Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, a doubly fed induction generator (DFIG) based wind power generation system can be decoupled into a positive- and a negative-sequence subsystem. Then, under small-signal operation, each subsystem is assumed to have linear characteristic in the frequency domain. Therefore, a single-in-single-out stability criterion can be applied for each subsystem to analyze the stability of the interconnected system consisted of the DFIG system and the grid. However, when there is frequency coupling in the DFIG system, this linear assumption may lead to wrong conclusion of the system stability analysis. The frequency coupling of DFIG can be caused by the PILL and the asymmetric rotor current control. In this paper, the frequency coupling characteristic of DFIG caused by these two causes will be studied, and an analytical model that describes the frequency coupling of DFIG will be developed and validated by simulations. Based on this derived model, the system stability and the influencing factors of frequency coupling will be analyzed.