The correctness of the sampled value differential protection behavior depends on the accuracy of the data window selection.Based on the hysteresis characteristics of the current transformer (CT),this paper analyzes instantaneous value and the rate of change of restraint current and differential current under the situation of busbar internal and external faults,and puts forward a data window selection method for sampled value differential protection:first select feather point of data window,then by the feather point to push down the starting point of data window,and RTDS test is carried out to verification the principle.The sampled value differential protection principle is not affected by the system frequency,the non-periodic current and CT transfer characteristics and so on,and it can still guarantee the reliability and rapidity in the complex fault condition,data window selection method is accurate and reliable,improves the sensitivity of sampled value differential protection action.