We demonstrate a novel all-fiber cavity ring-down (CRD) magnetic field sensing method that uses frequency-shifted interferometry, and does not require any optical pulse and fast electronics compared with conventional CRD schemes. The sensing element in the ring-down cavity is a fiber taper surrounded by magnetic fluid, whose refractive index varies as an external magnetic field is applied. Magnetic field strength measurement is successfully achieved within a range from 8 to 850 Gs. A resolution of 0.00105 ± 0.00003 dB/Gs is obtained in the approximately linear segment from 423.2 to 766.6 Gs. The sensing method is potential for sensing other physical and chemical parameters.