Abstract: The reflectivity calibration and correction of soft X-ray planar mirror is studied. Research is done on the 3W1B beamline of Beijing synchrotron radiation facility for mirrors with several materials, from 50 eV to 1 500 eV, with 40～120 mA beam current and 2 GeV storage ring electron energy. Variable-spacing planar grating on 3W1B generates higher harmonics necessarily, which reduces monochromaticity notably and brings about some errors. Though prefilter is used, limited by the grating dispersion effeciency and X-ray source internsity, there are still some higher harmonics during 300～750 eV. Therefore, the calibrated reflectivity must be corrected.