Large-scaled wind-farm connected to VSC-HVDC transmission system (WF-VSC system) may induce a new type of sub-synchronous oscillation which threatens the stability of the AC grid, this type of oscillation is the results of the interactive behaviours among the power electronic controllers through the AC grid, the interactive behaviours cause the impedance coupling characteristics of the WF-VSC system from sub-synchronous frequency range to twice fundamental frequency range, leading to significant inaccuracy when analyzing the impedance coupling characteristic using one-dimension impedance model. In this paper, the coupling characteristic is taken into account, and the sub-synchronous oscillation conditions of the WF-VSC are elaborated based on the coupled impedance model and the freuqency response characteristics of the integrated impedance, and the impacts of the oscillation conditions, such as the number of the wind turbines, the length of the AC transmission line, the output power ratio of the wind turbines, and the parameters of the VSC controllers, on the sub-synchronous oscillations stability are also analyzed. The simulation results verity the the effectiveness of the coupled impedance model and the analysing results.