The three single-phase on load transformer tap changing furnace transformer has the advantages of wide voltage controlled range and frequent voltage control. This paper presents the differential protection device of the furnance transformer should calculate the balance coefficient of the differential protection in real time. First, the gear information of 3 single-phase electric furnace transformer is converted to BCD code information through the OLTC gear controller; then the differential protection device accesses the converted BCD code through switch signal and decodes the information; finally, according to the transformer voltage total gears, each phase current transformer tap, low voltage side of the highest and lowest voltage nominal voltage, each phase current working voltage of low voltage side is calculated, combined with the transformer rated capacity, rated voltage of the high voltage side, the differential current calculation is adjusted in real time according to the current tap of the furnace transformer. This method solves the differential current electric transformer differential protection calculation problem is not correct. The engineering practice shows that the methods can increase the accuracy of the differential current calculation and significantly improve the differential current protection sensitivity.