Abstract: Adjustable-speed pumped storage unit (ASPSU) is an environmentally-friendly solution for primary frequency control in terms of its massive scale and reliability. Frequency response mechanism modeling and performance analysis of the ASPSU facilitate a better understanding and utilizing its merits for primary frequency control, which have been studied inadequately though. This study aims to derive a frequency response mechanism model of the ASPSU and to quantitatively analyze its superiority in frequency regulation performances to those of the thermal power unit and the fixed-speed pumped storage unit. Firstly, it is presented that the frequency response mechanism model of the ASPSU under the turbine and pumping modes. Then, it is figured out that the influence of droop coefficient on the frequency regulation stability of the three units respectively by the root locus method. Furthermore, the key dynamic indices are calculated when the three units contribute to the primary frequency control respectively. Finally, the model and performance analyses are validated by the PSCAD/EMTDC software. This study helps to make full use of the ASPSU to ensure the frequency stability of the new power system.