A comprehensive analysis on the structural features and working principles of cage-type current shunts is conducted. And a two-ports parameter matrix model is built and the AC-DC difference formula of the current shunt as a whole through cascade connection is presented. With this mathematic model, main factors influencing the AC-DC difference are given. Based on theoretic analysis, a new method to improve the AC-DC difference performance is presented by increasing the precision of group difference between ribs. And test results before and after the method on a 20A rated current shunt has verified the effectiveness of this method which guides the direction for future research.