Accurate measurement of DC high current has been widely used in many fields. The improvement of calibration accuracy of DC high current transformer has been restricted by many factors such as magnetic field interference and calibration technology. This paper analyzes the existing problems of the self-calibration method of self-balancing DC high-current transformer and proposes an equal flux self-calibration method. The error transfer formula is deduced and the standard uncertainty is evaluated by analyzing the principle and error of flux self-calibration methods of self-balancing DC high current transformer. The results show that this method has the advantages of fixed primary winding flux leakage effect, small calibration error uncertainty and fast expansion range, which can achieve high accuracy and high efficiency expansion of current ratio standard under actual working conditions and is more able to meet the current needs of DC current transformer calibration as well.