We reported the modeling result of selectively magnetic fluid infiltrated dual-core photonic crystal fiber based magnetic field sensor. Inside the cross-section of the designed photonic crystal fiber, the two fiber cores filled with magnetic fluid () form two independent waveguides with mode coupling. The mode coupling under different magnetic field strengths is investigated theoretically. The sensitivity of the sensor as a function of the structural parameters of the photonic crystal fiber is calculated. The result shows that the proposed sensing device with 1 cm photonic crystal fiber length has a large sensitivity of .