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We present a high-sensitivity fiber optic soil moisture sensor based on side-polished multimode fibers and lossy mode resonance (LMR). The multimode fibers (MMFs), after side-polishing to form a D-shaped structure, are coated with a single-layer SnO2 thin film by electron beam evaporation with ion-assisted deposition technology. The LMR effect can be obtained when the refractive index of the thin film is positive and greater than its extinction coefficient and the real part of the external medium permittivity. The D-shaped fiber optic soil moisture sensor was placed in soil, allowing moisture to penetrate into the thin film microstructure, and it observed the resonance wavelength shift in LMR spectra to measure the relative humidity change in soil. Meanwhile, an Arduino electronic soil moisture sensing module was used as the experimental control group, with soil relative humidity ranging from 10%RH to 90%RH. We found that the D-shaped fiber with a residual thickness of 93 μm and SnO2 thin film thickness of 450 nm had a maximum sensitivity of 2.29 nm/%RH, with relative humidity varying from 10%RH to 90%RH. The D-shaped fiber also demonstrates a fast response time and good reproducibility. [ABSTRACT FROM AUTHOR]