The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the most sensitive ground-based, singledish radio telescope on Earth. However, the original H I spectra produced by FAST are affected by standing waves. To maximize the power of FAST for high-sensitivity observations, we proposed an algorithm that combines fast Fourier transforms and extreme envelope curves to automatically correct the baselines of FAST H I spectra and remove standing waves from the baselines. This algorithm can reduce the amplified noise level caused by standing waves to a near-ideal level without losing signals or introducing false signals. The root mean square of the average baseline reaches ∼8 mK, approaching the theoretical sensitivity of an H I spectrum produced by FAST for an integration time of 335 minutes, i.e., ∼6 mK.

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