This paper analyzes the defects of the 2nd harmonic inrush current discriminating method that is traditionally used in transformer differential protection. A fuzzy recognition method is introduced to improve it. The improved 2nd harmonic fuzzy recognition criterion has better results by making use of the adaptive criterion， canceling a single line style of the“across the board” threshold. The proposed criterion can better match the relationships among three differential currents by using the inverse time and delay threshold features in the integrated membership degree calculation. This criterion is simple， scientific and reasonable in principle， and easy to implement. It has a great progress both in the operational speed and anti-maloperation.