In the electric vehicle charging station DC microgrid (DC-MG), disturbances from distributed generations and loads are easy to cause DC bus voltage fluctuations and even result in system instability. Then, the virtual DC machine (VDCM) control strategy is introduced to enhance the system damping and inertia. However, it is found that this method has a poor robust against passive network variation and the system is at risk of high-frequency resonance. For this issue, based on the typical topology and control strategy of charging station DC-MG, this paper adopts impedance analysis method and establishes the small-signal DC impedance models of VDCM, transmission cable and constant power load (CPL). Besides, the impedance characteristics of each subsystem and their interaction are analyzed. The negative damping characteristic of high-frequency band was explained and the wideband oscillation mechanism is revealed. Finally, a simulation based on MATLAB was used to verify the correctness of the wideband oscillation mechanism proposed in this paper.