A high-sensitivity magnetic field sensor based on a dual-core photonic crystal fiber has been designed with an extremely short device length of 2000 μm in this paper. The two cores of the fiber are separated by one air hole filled with magnetic fluid. The sensitive properties are investigated by the full-vector finite element method. Simulation results illustrate that the highest sensitivity can reach -442.7 pm/Oe in the magnetic field strength range of 30-520 Oe. The photonic crystal fiber filled with magnetic fluid, serving as an excellent platform for magnetic field sensing, has great potential applications in complex environments, remote sensing, and real-time monitoring fields.