As the unique power-discharging channel for high-speed trains, the 'train-rail' mobile coupling grounding system plays a vital role in sending the traction power back to the terrestrial substations, through the electrical contact between the rotating wheel and the steel rail. The whole system is composed of two kinds of grounding modes: working grounding is in charge of draining the traction power into the rail track; the protective grounding provides the reference grounding potential for the vehicle-mounted equipment. Different from the fixed terrestrial grounding technique normally implemented in power system, when the relative motion occurs between the train and the rail, both the 'catenary-to-train' and 'train-to-ground' impedances vary transiently, which may lead to the current flowing through each grounding wheel varying unpredictably. Herein, the 'catenary-train-rail' mobile coupling grounding model was built based on the equivalent circuit modeling technique for evaluating the transiently-varying tendency of the grounding impedances and currents flowing through each grounding wheel. To cope with different operational conditions, the adjustable grounding impedance methodology is firstly proposed for adapting the unpredictable variation of the grounding impedance, when the relative motion occurs between the train and the rail. Ultimately, the validity of the novel grounding system applying the adjustable grounding impedance technology is verified under different operational conditions, with the consideration of both TB current and voltage.