A new second-order harmonics restraint principle is proposed to solve the problems in the existing methods. The threshold values of second-order harmonic blocking inrush current are adjusted according to the breaker positions of transformer and the differential currents. By monitoring internal faults and magnetizing inrush currents during the process of no-load switching and voltage restoring after fault clearances， the magnetizing inrush current can be judged more precisely without causing resistive operations and maloperations of protection. The new method has the function of real-time tracking and overcomes the shortage of single setting point that cannot realize reliable restraints. The reliability and sensibility of the new method are demonstrated by testing results.