For the calibration of broadband standard current transformers, the method of measuring deviation is usually used in the frequency band near 50 Hz. However, at frequencies as low as 5 Hz and as high as 2.5 kHz, due to the principle of using flux gate sensors in most standard transformers, it is not possible to correspond one-to-one with the transformation ratio of the tested transformer. Therefore, a synchronous sampling digital comparison method must be used. This paper analyzes the broadband characteristics of coaxial shunt, a broadband transformer calibrator suitable for self calibration of coaxial splitters has been designed to achieve error calibration of broadband standard current transformers. The calibration and traceability of the splitter are achieved through the combination of a multifunctional calibration source and a high-precision digital multimeter. This paper provides a comprehensive calibration plan and detailed test data, and conducts a scientific analysis of the calibration uncertainty in each link, demonstrating the rationality and accuracy of the calibration method. This method has great practical significance for the measurement work of the currently operating 20 Hz demonstration power grid of Zhejiang Electric Power Company.