The invention discloses a method for judging the startup of an interphase current mutation amount through a floating threshold. The realization method is that the floating value is increased on the basis of the fixed value of the starting current of the sudden change amount, and the starting threshold of the sudden change amount of the current between phases is taken as the starting threshold value. When the instantaneous value of the interphase current abrupt change is greater than the threshold of the interphase current abrupt change, the interphase current abrupt change element is allowed to start. Using the method of judging the sudden change of current between phases by floating threshold, the frequent start-up of the protection device can be prevented when the error of the system frequency relative to the power frequency exceeds +/- 2 Hz during operation.