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A novel and practical magnetic field sensor based on optical fiber optics is proposed in our work. We first demonstrate magnetic sensing with the structure that single-mode optical fibers are fused with capillaries in parallel in an experiment. We clearly show the aggregation and arrangement variation with the magnetic field of magnetic nanoparticles in capillaries. Based on the tunable effective refractive index of optical modes in a waveguide structure of a sensor, the optical properties and sensing mechanism in the sensing structure were simulated and further analyzed. We achieved the detection of a space magnetic field, including intensity and its direction. We obtained that the sensitivity of a sensor is 112 pm/mT, presenting good performance in the same kind of optical fiber magnetic field sensor.