The improvement of stable and reliable operation of giant magneto-resistance（GMR） based active electronic current transformer is one of key problems. An intelligent GMR based DC current transformer is designed. A self-calibration module， which realizes the on-line calibration of GMR sensor， is designed and embedded in the high-voltage side circuit. Then， the self-modification method， which is utilized to solve the non-linear problem of GMR sensor， is proposed. Finally， the self-diagnosis method of the high-voltage side circuit is introduced， and the backup microcontroller is used to improve reliability. The performance of the intelligence method is verified by testing results. The stable and reliable operation of the GMR based DC current transformer is improved significantly.