A high sensitivity magnetic fluid (MF) filled side-hole fiber magnetic field sensor based on surface plasmon resonance (SPR) is designed and investigated. Within the side-hole fiber, the water-based MF is injected into the two side holes, and both of the holes are coated with gold to realize the measurement of the magnetic field by using the SPR effect. The refractive index of the MF changes with different external magnetic fields, resulting in the resonant wavelength moving to other wavelengths. Furthermore, when the external magnetic field increases from 30 to 210.9 Oe, the magnetic field sensitivity can reach to 1.063 nm/Oe.