Currently， there is only over-current protection for large capacity furnace transformer and without longitudinal differential protection（LDP） at home and abroad， in order to remedy the lack development of it， a LDP of furnace transformer is discussed. An optimal compensation equation for LDP of furnace transformer is put forward， which is different from that of traditional transformer. The particular protection problem during the breaker closing of furnace transformer is analyzed and its solution is also presented. As the gradient setting values for longitude difference in two sides of furnace transformer are small， the restraint current calculation of LDP in three sides of furnace transformer is also different from that of traditional LDP of transformer. The characteristic of the current for LDP is analyzed when the low voltage side of furnace transformer has particular wiring and grounding fault. The characteristic for low voltage start of backup protection of furnace transformer is also analyzed. The protection device developed has been put into production.