In this paper,an all-fiber magnetic field sensor based on magnetic f luid (MF) and the single mode-multimode-singlemode (SMS) structure,using a piece of no-cor e fiber (NCF) as the multimode waveguide,is proposed.The SMS structure is sealed in a capillary f illed with MF.The change of the strength of the external magnetic field will cause a wavelength-shif t dip or peak wavelength attenuation on the transmission spectrum and then be detected and measured.An al l-fiber magnetic field sensor with sensitivities up to 78pm/Oe and 0.129dB/Oe,respectively,was fabricated and experimentally investigated.Besides being used as a magnetic field sensor,the device can also be used as a magnetic-driven tunable optical attenuator or modu lator.