Following the discussion of traditional 2nd order harmonics restraint principle and its deficiency， the paper analyzes variation characteristics of the fundamental component and 2nd order harmonics component of differential current when a normal transformer is energized and when a faulty transformer is energized. Based on the characteristics， a new adaptive inrush current restraint principle is proposed. The method judges the real-time operation state of a transformer and ensures secure blocking and dependable activation of differential protection operation. The method solves the conflict of inrush restraint and correct and fast protection operation， and improves restraint sensitivity， which ensures that the restraint takes effect only when inrush current occurs and the correct and fast protection operates when any internal fault happens.