Abstract: The input power of the conventional constant-speed pumped storage unit cannot be continuously adjusted under the pumping mode, and it does not have the ability to participate in frequency control of the grid. Doubly-fed variable speed pumped storage unit is a new type of hydropower system. Because the speed can be adjusted continuously, the input power of the unit under pumping mode can be adjusted more widely on the premise of ensuring the static head. In order to explore the potential of the unit in participation in grid frequency control, a simplified electromechanical transient model of the unit in pumping mode was first established. Based on the operation characteristics of reversible pump-turbine in pumping mode, the dynamic characteristics of the fluid in penstock were considered, and the additional friction effect of guide vane was taken into account. Then, the essential reason why the pumping mode unit had the ability to carry out frequency control from the perspective of hydraulic was revealed. Next, the mechanism of the unit in improving the frequency characteristics of the grid was expounded, and the recommended range of controller parameters was given. Finally, the simulation and experimental results verified the capability of the proposed pumping mode unit in improving the frequency characteristics of the grid.