The invention discloses an auxiliary device for the field calibration of a high-transformation-ratio current transformer used in a GIS. The auxiliary device is characterized by comprising a power frequency power supply which can output 380 VAC, wherein the power frequency power supply is connected with the primary side of a current booster; the auxiliary edge of the current booster is connected with the current transformer in the GIS; first capacitor banks are connected in series between the auxiliary edge of the current booster and the current transformer, and second capacitor banks are connected in parallel between the auxiliary edge of the current booster and the current transformer; switches are arranged among the first capacitor banks; switches are arranged among the second capacitor banks. The auxiliary device disclosed by the invention solves the problems that the current booster is high in capacity, the voltage regulator is high in capacity, the reactive component compensation efficiency of a loop is low and the increasing of the total current is difficult during the field calibration of the high-transformation-ratio current transformer in the GIS, and the test working strength is lowered.