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A simple, low-cost, and high-sensitivity optical fiber displacement sensor is presented and fabricated by concatenating two core-offset joints (COJs) with a separation length of 13 mm using a commercial fusion splicer. The two COJs construct an in-line Mach-Zehnder interferometer (MZI). When the MZI is bent to have a bending radius of ~ 17 mm, the interference pattern of the MZI depends strongly on the bending radius. According to this character, we scheme the bent MZI as a displacement sensor in two ways providing two different bending types of the bent MZI. The displacement sensitivities obtained are up to 0.835 and 0.227 nm/μm in the large displacement variation ranges of 350 and 1000 μm, respectively.