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In this article, a portable gas charging device for SF6/N2 gas-insulated current transformer is developed to meet the demands of on-site modification of SF6/N2 gas-filled current transformer. This device has the functions of SF6 retrieving, vacuum pumping, mixed gas filling, and mixed gas ratio detection. This device adopts the partial pressure controlling method to realize the gas mixture ratio. By filling the mixed gas successively, the required pressure value is finally achieved without presetting the total volume of inflating gas. The operation process is simple and intuitive, and the operation efficiency is high. The built-in temperature detection and pressure detection module can carry out real-time detection and temperature correction for the charging pressure of mixed gas, improving the accuracy of the charging device. A buffer cavity is designed to speed up the homogenization of the mixed gas in the equipment cavity. The gas in the equipment cavity is poured into the buffer cavity repeatedly to force it to mix quickly, which reduces the time required for the gas to be set even and improves the charging efficiency. The experiment results show that the mixing ratio accuracy of the charging device can be controlled within 1%, and the pressure deviation can be controlled within 0.02 MPa. With the gas charging device, two current transformer on-site modifications have been successfully conducted.