In order to solve the problems occurred in field error measurement of current transformer in UHV AC transmission Gas-insulated Metal-enclosed Switchgear (GIS), an error measurement method of self calibration CT realized by equal ampere-turns is proposed. Double secondary windings with the same structure are arranged in the CT’s core to realize error self calibration of CT. In normal working condition, the double secondary windings are connected in parallel; in the error test, the double secondary windings are separated, through inputting current to one of the secondary winding (equivalent to primary winding), the magnetic potential under the equivalent normal working condition is constructed, and the measurement of CT’s error can be realized by measuring the other winding’s current (equivalent to the secondary winding) under the self calibration circuit. Compared with the traditional comparison method, it can effectively reduce the requirements of large-scale test equipment and power supply in field work, and reduce the workload of test and operation and maintenance. The simulation and experiment of the self calibration CT used in the engineering demonstrate the effectiveness of the method. This work is supported by Science and Technology of State Grid Corporation of China (No. GY71-15-034).