The invention relates to the pumped storage power generation field, and specifically relates to a high frequency isolation cascade AC excitation device for a variable-speed pumped storage power generation system. The high frequency isolation cascade AC excitation device comprises a rotor side grid connected switch module, a transformer module and a three phase cascade type isolation double back-to-back H bridge module, wherein the rotor side grid connected switch module is connected with the transformer module; and the transformer module is connected with the three phase cascade type isolation double back-to-back H bridge module. The high frequency isolation cascade AC excitation device for a variable-speed pumped storage power generation system has a frequency conversion operation capability and can satisfy the excitation requirement for speed change operation of a pumped storage generator set. The high frequency isolation cascade AC excitation device for a variable-speed pumped storage power generation system has a bidirectional energy flowing capability, and can satisfy the excitation requirement for generating condition and electric condition operation for the pumped storage generator set. The high frequency isolation cascade AC excitation device for a variable-speed pumped storage power generation system has a cascade feature, and can realize access of the high voltage grade of an AC excitation current transformer while avoid using a special high overpressure resistant semiconductor switch device.