In recent years, along with the wide applications of the high voltage direct current (HVDC) technology based on modular multilevel converter (MMC), the broad-band oscillations related to the MMC have also been frequently observed and drawn a lot of attentions. Therefore, focusing on the existing Luxi VSC-HVDC transmission system which has been experiencing this kind of oscillations, which have been taken into account the actual configuration of the Luxi VSC-HVDC transmission system, and the wind-band coupling impedance model of the named system is derived. This model demonstrates the dynamic charateristics of each control loops and their impacts on the impedance characteristics. It also reflects the coupling effects of the harmonic components. The electro-magnetic model of the Luxi VSC-HVDC transmission system has also been implemented to obtain the impedance-against-frequency curve by disturbance-identification method. The comparison between the impedance-against-frequency curve and impedance model has also proven the consistence of both model from a few Hertz to 2 000 Hz.