The invention discloses an EHV GIS extra-long bus full voltage AC voltage withstand test device comprises a variable-frequency power supply FC, a reactor L1, a reactor L2, a voltage measuring device V, a test transformer Tr, a voltage dividing capacitor C1, a voltage dividing capacitor C2 and a sample capacitor Cx. The input end of the variable-frequency power supply FC is connected with a three-phase power supply. The output end of the variable-frequency power supply FC is connected with the test transformer Tr. The test transformer Tr and the reactor L1 are connected in series and then are connected in series with the reactor L2. The voltage capacitor C1 and the voltage dividing capacitor C2 are connected in series and then are connected to the reactor L2. The voltage measuring device Vis connected to the voltage dividing capacitor C2. One end of the sample capacitor Cx is connected in series with the voltage dividing capacitor C1, and the other end of the sample capacitor Cx is grounded. In combination with an actual situation, it is difficult for the capacity of a transformer station test power supply to meet a test requirement. The capacitive current of a bus system is compensated by the current of an inductor, and a 800kV GIS extra-long bus full voltage AC voltage withstand test device with a new structure is designed in combination with theoretical calculation.