In this work, a magnetic field sensor based on a no-core-three-core-no-core fiber structure was proposed and fabricated. Two segments of 2-mm no-core fiber were spliced at both ends of a 50-mm three-core fiber, and the structure was inserted into a 70-mm-long capillary tube; in addition, magnetic fluid was injected into the capillary tube using a needle, such that the no-core-three-core-no-core structure was completely immersed in the magnetic fluid. The no-core fiber was used to excite the cladding mode of the three-core fiber and achieve inter-mode interference. The magnetic field intensity can be determined by measuring the wavelength shift of the transmission spectral dip or by detecting the intensity loss of the transmission spectral dip. The experiment shows that the wavelength shift of the interference spectrum near 1 606 nm has a linear relationship with the change in intensity of the magnetic field, with a corresponding wavelength shift sensitivity of 68.57 pm/mT when the magnetic field intensity is within the range of 8-16 mT. Within the same range of magnetic field intensity, the intensity loss of the interference spectrum near this wavelength shows good linearity, and the corresponding intensity sensitivity is 0.828 7 dB/mT. The proposed sensor structure has the advantages of a simple structure, high sensitivity, and low cost, with potential application in magnetic field detection.