Abstract: In this paper, the preprocessing method for the visibility functions in synthetic aperture radiometer is proposed, including the correction on the AD bias, the inversion of the analog correlation from the raw 3-level digitized correlation, the correction of the quadrature error, and the compensation of the fringe washing effects. The adjustment of the analog correlation from the raw 3-level digitized correlation is particularly introduced, which is the first time that the method is applied to the calibration of the earth observation domain. The Monte-Carlo simulation in time domain on the preprocessing method for the visibility functions is performed. The correctness of the preprocessing algorithm is tested and verified by the Geostationary Interferometric Microwave Sounder system. The bias of the result after adjusted is better than 0.1% of the true value, which could proof that the preprocessing algorithm can be used in the calibration of the Geostationary Interferometric Microwave Sounder for spaceborne system. And the result can easily be used in the inversion of the brightness temperature imaging.