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A novel and in-line fiber low-frequency vibration sensor based on Butterfly-Shape Mach-Zehnder Interferometer (BSMZI) is proposed and demonstrated experimentally. The sensing element is a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs). The frequency measured by the Butterfly-Shape vibration sensor can well match with the applied frequency, and the presence of the higher harmonics can verify that the sensor does not distort the measured vibration signal. The maximum relative error of vibration frequency is 0.27%. The intensity of vibration signal (of the fiber sensor) is linearly proportional to the driving power of vibration platform. The vibration measurement sensitivity is 13.575 dB/w in the range of 0.1 w to 0.5 w under a constant vibration frequency of 50 Hz, and the linear fitting coefficient is 0.9556. The proposed sensor has a negligible temperature sensitivity in the range of 30.3 °C–69.5 °C. Because of its compact structure, easy preparation, low production cost, high frequency measurement accuracy of low-frequency vibration, and good linearity of vibration sensing, the proposed BSMZI sensor has potential application prospect in low-frequency vibration sensing field. [ABSTRACT FROM AUTHOR]