A theory for magnetic-temperature coupling effect generated in a Fiber Optic Gyroscope (FOG) under the combined action of magnetic and temperature fields was proposed. The magnetic-temperature coupling in FOG originates from the interaction of the magnetic field, fiber twist, birefringence caused by thermal stress, and the intrinsic and bending birefringence of the fiber. The cross-coupling changes with temperature. When the polarization maintaining fiber has a diameter of 250 μm, beat length of 3 mm, length of 1 600 m, twist rate of 1 rad/m, and optical source wave length of 1550 nm, the maxim degree of magnetic-thermal coupling generated by a 1 mT radial magnetic field within the temperature range of 20℃ to 60℃ is 6.796%.