Magnetic fluids are widely used in the field of optical fiber magnetic field sensing as magnetic sensitive materials due to the adjustable refractive index. Optical fiber magnetic field sensor based on magnetic fluid has the advantages of high sensitivity and light weight, but the magnetic field measurement range of this kind of sensor is affected by the saturation magnetization of magnetic fluid, which limits the high magnetic field sensing range. Some studies have shown that magnetic hydrogels have the characteristics of high sensitivity to magnetic field, high saturation magnetization, high magnetic responsiveness and super magnetic similarity with magnetic fluids. In this paper, it is proposed to replace the base liquid of magnetic nanoparticles of magnetic fluid with hydrogel material to prepare magnetic hydrogel, hoping to seek a higher level of saturation magnetization, and make prospects for further improving the magnetic field measurement range of optical fiber sensors. A polyvinyl alcohol/ferric oxide (PVA/Fe₃O₄) magnetic hydrogel was prepared by the composite method, and used a fiber end face reflection-based measurement method to test the refractive index variation of the magnetic hydrogel under different magnetic fields, which proved that it had magnetic-induced refractive index variation characteristics and could be applied in the fiber magnetic field sensing structure based on refractive index variation. Based on this research, a cone-shaped fiber sensing structure based on magnetic hydrogel was designed. Single-mode optical fiber is pulled into a cone shape by melting and pulling method, and magnetic hydrogel is wrapped in the cone area and waist area of the cone-shaped optical fiber. The input light in the fiber will generate evanescent wave when it is in the conical transition region, which will excite the higher-order cladding mode in the fiber. After the fundamental mode and cladding mode are transmitted in the waist region of the tapered fiber, mode coupling and mode interference will occur when entering another tapered region. When the intensity of the magnetic field applied in the external environment changes, the refractive index of the magnetic hydrogel will change. In this experiment, we change the refractive index of the magnetic hydrogel by applying magnetic fields of different intensities to the sensing element of the tapered fiber wrapped by the magnetic hydrogel. The spectral change of the output light of the tapered fiber is detected by using the evanescent wave in the tapered region of the tapered fiber, which is sensitive to the refractive index of the external environment, to study the magnetic field sensing characteristics of this structure. The experimental results show that the sensitivity of wavelength shift is 86.42 pm/mT within the range of 6.4~22.6 mT, when the magnetic particle concentration is 2.1% at a constant temperature of 22 ℃. And also the sensitivity of wavelength shift is 51.42 pm/mT within the range of 5.5~30 mT, when the magnetic particle concentration is 2.9%. From the point of view of optical fiber magnetic sensing measurement, magnetic hydrogel has high application value and deserves further research. This paper also proposes that the detection sensitivity can be improved by selecting more appropriate dispersants to improve the dispersion uniformity of magnetic nanoparticles in hydrogels, and by improving the structure of optical fiber sensing elements. During the preparation of magnetic hydrogel, we can further improve the magnetic field sensing range and expand the application of magnetic hydrogel in the field of optical fiber magnetic field sensing by optimizing the freezing and thawing times and the concentration of magnetic nanoparticles.