The invention discloses a differential protection device based on a Faraday magneto-optical effect, relating to a differential protection device, belonging to the field of relay protection of a power system. According to the differential protection device, the problems that large error components can be generated in differential operation and an internal fault and an external fault inside a section can not be accurately judged in an existing differential protection device. The differential protection device comprises a light source, a first multimode fiber, a second multimode fiber, a first optical current sensor, a polarization maintaining optical fiber, a second optical current sensor and an electronic processing unit; the first optical current sensor comprises a first multimode optical fiber collimator, a polarizer, a first Faraday magnetic-optical element and a first polarization maintaining collimator; and the second optical current sensor comprises a second polarization maintaining optical fiber collimator, a second Faraday magnetic-optical element, a polarization analyzer and a second multimode optical fiber collimator. The differential protection device is used in relay protection of the power system.