A kind of magnetic field sensing solution is proposed based on 3-D microstructure machining of fiber. The femtosecond laser with wavelength of 780 nm is employed to ablate a spiral microstructure into cladding of fiber Bragg grating (FBG), and the magnetostrictive film (TbDyFe) is deposited in the microstructure by magnetron sputtering process to form a new fiber optic magnetic field sensor. The microstructure can improve the axial retractility of fiber and enhance the surface area of the thin film deposition in order to enhance the sensitivity of probe. The theory of enhancing sensitivity from spiral microstructure is established and the preporation methods and techniques of the fiber optic magnetic field sensor probe are described. The magnetic response results of several probes with different parameters are demonstrated. It is shown that from the experimental results, the sensing probe with pitch of 50 μm is most sensitive to magnetic field. In ideal case, the sensitivity of the probe with microstructure can be enhanced nearly five times as high as that with non-microstructured standard FBG.