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Nowadays, optical fiber sensors are extensively being explored for the detection of magnetic fields. Against this backdrop, a highly sensitive magnetic field sensor based on the combination of magnetostrictive material and Vernier effect is proposed and experimentally demonstrated. The proposed sensor consists of two Fabry-Perot interferometers (FPIs), each comprising two sections of hollow-core fiber as FP cavities and a single-mode fiber (SMF) as a separator. One of the FPIs, which is bonded to the magnetostrictive material, undergoes further modulation by a high-frequency CO2 laser to enhance its sensitivity. Both simulated and experimental results demonstrate that the proposed sensor exhibits significant spectral response to the extension effect of the material under magnetic field variation. Within the range of 0–11.34 mT, the highest sensitivity achieved is 1.10 nm/mT, while demonstrating excellent temperature stability and repeatability. With its low-cost, simple structure, easy fabrication process and high sensitivity, this proposed sensor exhibits great potential for magnetic field measurement.