The invention discloses a setting method and device of a setting value of a longitudinal differential protection, comprising the following steps: obtaining a primary rating value of a first current transformer, a primary rating value of a second current transformer and a primary parameter of a line voltage transformer; calculating a first maximum load current of the line according to a smaller value of a primary rating of the first current transformer and a primary rating of the second current transformer; according to the primary parameters of the line voltage transformer, selecting the natural power of the line flexibly; calculating a second maximum load current of the line according to the natural power of the line; taking a small value of the first maximum load current and the second maximum load current as the setting value of the longitudinal differential protection and writing the same into the differential protection device. The setting method of the setting value of the longitudinal differential protection provided by the embodiment of the invention is simple and clear, can ensure the correctness of the setting value of the CT broken line differential current, and the setting device can write the correct setting value into the differential protection device, thereby enhancing the safety of the transmission line.