The criterion of transformer magnetizing inrush current based on positive sequence component of differential current harmonics can identify inrush and internal fault effectively. In order to further improve the blocking reliability in magnetizing inrush current cases and operation speed in internal fault cases of differential protection, terminal voltage and its mutation in positive sequence component of fundamental harmonic are analyzed respectively in magnetizing inrush cases and internal fault cases. According to their different characteristics, an adaptive control factor Q is determined, which is added to the magnetizing current criterion based on positive sequence of differential current harmonics, then a new adaptive criterion of transformer magnetizing inrush current is proposed. A large number of EMTP simulations and dynamic simulation experiments show that the new criterion can automatically decrease the threshold setting of restraint in inrush cases, so the reliability of inrush blocking is improved, and it can also automatically increase the threshold setting of restraint in internal fault cases, so the operation speed during internal fault conditions is accelerated.