The invention relates to a transformer differential protection method and device based on a current transformer disconnection. The transformer differential protection method comprises the steps of judging whether a current transformer disconnects; if the current transformer disconnects, judging whether differential protection is started; if the differential protection is started, instantaneously determining that the current transformer disconnects, or otherwise, delaying to determine that the current transformer disconnects; after determining that the current transformer disconnects, judging whether disconnection locking differential protection is used; if the disconnection locking differential protection is not used, alarming the current transformer disconnection and opening the differential protection, and if the disconnection locking differential protection is used, judging whether a differential current is greater than a set value; if the differential current is greater than the set value, alarming the current transformer disconnection and opening the differential protection; and if the differential current is not greater than the set value, alarming and locking the differential protection. The transformer differential protection method and device related by the invention can reliably detect whether the current transformer disconnects; and the problems that the differential protection is likely to be mistakenly operated after the current transformer disconnects, and the differential protection refuses operation when a failure reoccurs are effectively solved.