By analyzing a misoperation accident of a current differential protection using fiber communication channel on a high voltage transmission line， it is discussed that residual magnetic flux intensity in current transformer is created in large quantity by transient non-periodic current component. The residual magnetic flux intensity makes the starting work point of current transformer change on B-H curve， and hence the magnetic saturation state is aggravated in shorter time， which is the main reason to cause the misoperation accident of differential protection. The relationship between error curve of current transformer and ratio restraint coefficient is analyzed. The analytical result shows that it is inappropriate to study characteristic of differential protection piecewise only based on stalling current amplitude and to increase retain coefficient blindly without taking account of the influences of residual magnetic flux intensity. It can be concluded that it is a more reasonable way to take the magnetic induction intensity in current transformer as the basis to study the sectional ratio restraint characteristic of current differential protection. Finally， a new type of practical scheme is produced.