In China, the high-penetrated doubly-fed induction generators (DFIG) wind turbine (WT) is undergoing the centralized collection and long-distance line-commutated converter (LCC) high voltage DC (HVDC) transmission. In such high-penetrated renewable generation and transmission system, the dynamics of power electronics become dominant over the conventional synchronous generator so that its modeling and stability analysis needs re-examining. However, the interaction among interconnected wind farm, and LCC-HVDC, and weak grid remains unclear but may easily incur the stability issue. This paper firstly builds the wind farm model by aggregating each WT and collection transmission with the consideration about the frequency coupling. The double Fourier transform is further used to derive the mapping function in frequency-domain relates the current and voltage harmonics between DC and AC sides of the 6-pulse LCC converter, based on which the analytical 6-pulse LCC-HVDC impedance from AC side can be theoretically developed. The developed analytical impedance of DFIG wind farm with LCC-HVDC transmission system is verified by the comparison with the frequency sweeping simulation results and the oscillation reproduction in MATLAB/Simulink. The major parameter influence on system impedance characteristics is finally analyzed and compared in detail.