The invention relates to a differential protection device based on an optical current transformer, and belongs to the field of power system relay protection. The differential protection device aims to solve a problem that a complete synchronizing device must be configured in a transformer substation when an existing relay protection device needs to transmit current information detected by a current transformer to a digital protection device in a synchronous sampling mode. Two photoelectric conversion circuits of the differential protection device respectively receive current information measured by a local-size optical current transformer and an opposite-side optical current transformer, the current information is converted into electric signals, the electric signals are respectively processed through two pre-filtering amplifying circuits and then transmitted to a microprocessor, a differential protection judgment result is acquired, and the judgment result is transmitted to an execution unit through the first photoelectric conversion circuit via an I/O port; and the microprocessor sends acquired detected current information to a display circuit to display through the I/O port, and the detected current information is converted into optical pulse signals through the second photoelectric conversion circuit at the same time and transmitted to a merging unit. The differential protection device is applied to relay protection of a power system.