This paper presents a novel method to eliminate the frequency coupling phenomenon of DFIG system based on complex vector phase locked loop (PLL). It is well known that the impedance model of DFIG system considering conventional PLL is a multi-input-multi-output (MIMO) system which is complicated to analyze the stability with weak grid and difficult to design control parameters. In this paper, a sequence impedance model of DFIG system is built based on complex transfer function. The DFIG system is simplified as a single input single output (SISO) system by applying the complex vector PLL. Thus, the DFIG system can eliminate the frequency coupling phenomenon which is better for designing the parameters of controllers and doing the stability analysis. Furthermore, the effect of complex vector PLL on the impedance of DFIG is analyzed. Based on the sequence impedance model, the stability of DFIG connected to weak grid is analyzed just by the ratio of grid impedance and DFIG impedance. Frequency scan simulations are also carried out in Matlab Simulink to verify the sequence impedance model.