The invention relates to the field of pumped storage power generation, in particular to a cascade alternating-current excitation device for a variable-speed pumped storage power generation system. The cascade alternating-current excitation device comprises a rotor-side grid-connected switch module, a multi-winding transformer module and a three-phase cascade back-to-back H bridge module, wherein the rotor-side grid-connected switch module is connected with the multi-winding transformer module; and the multi-winding transformer module is connected with the three-phase cascade back-to-back H bridge module. The cascade alternating-current excitation device has the advantages that the cascade alternating-current excitation device has variable-frequency operation capacity, can meet the excitation requirements of operation and variable-speed operation of a pumped storage generator set, has bidirectional energy flowing capacity, and can simultaneously meet the excitation requirements of operation of the pumped storage generator set in a power generation working condition and an electric working condition.