In order to meet the high requirements of the data measurement in the inertial confinement fusion(ICF) experiment, experimental instruments must be developed in the direction of absolute measurement, and the requirements for calibration are also higher. Beijing Synchrotron Radiation Facility (BSRF) provides a good X-ray source for calibration in China. The characteristics of the light source, such as the numerical study of the high-order harmonics content, are very important for the calibration work and the precise diagnosis of ICF. By measuring the current of a standard detector with or without a filter on a soft-X-ray source of beam line 4B7B in BSRF, the transmittance curves of different filters for monochromatic X-ray were studied and a fitted theoretical transmission curve was established. According to this, the proportion of second harmonic in the monochromatic light source was calculated. The results show that the second harmonic in the soft X-ray energy segment is mainly concentrated in the 180-300 eV and 450-800 eV, the share is mostly 15% below, the maximum can reach about 25%. Then, the calibration of the flat-response filter transmittance and the X-ray diode sensitivity was corrected using the calibration results. The revised results are consistent with the theory, which greatly improves the precision diagnosis capability of the diagnostic equipment. The complete theoretical model and experiment results mutual verify, showing that the filter-based technology for evaluating proportions of high-order harmonics is currently mature and has broad application prospects.