Instrument line shape function ( ILS) is one of the important physical characterization parameters of the fourier spectrometer, which affects the accuracy of the spectrometer. With the high-precision demand for remote sensing applications such as space measurement and atmospheric detection, howto measure and update the ILS of the on-orbit spectrometer in real time is an important means to improve the accuracy of on-orbit ultrahigh resolution spectrometer. This paper used Fourier spectrometer as an example, according to the principle of ILS, used the characteristics of the measured solar spectral calibration data of the on-orbit ultrahigh resolution spectrometer, which have independent Fraunh9 fer line and less influenced by atmospheric aerosols, to monitor and update the ILS of the on-orbit ultrahigh resolution spectrometer. In this paper, the Kurucz solar spectrum model is used as the reference spectrum, the characteristic peaks of the measured solar calibration spectra and reference spectra are selected in the corresponding band, by adjusting the slit model of the spectrometer, the characteristic peak residuals are iteratively compared to calculate the ILS parameters of the instrument. Finally, after making a convolution between the updated ILS and the theoretical limb spectrum, we compared the results with the measured limb calibration spectrum. The error range is-6% ~ 8%. These results showthat thismethod can provide a reference for supervising and updating the ILS of on-orbit ultrahigh resolution spectrometer.