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Observing a telluric standard star for correcting the telluric absorptionlines of spectrum will take a significant amount of precious telescope time,especially in the long-term spectral monitoring project. Beyond that, it'sdifficult to select a suitable telluric standard star near in both time andairmass to the scientific object. In this paper, we present a method ofcorrecting the telluric absorption lines by combining the advantages of longslit spectroscopy. By rotating the slit, we observed the scientific object anda nearby comparison star in one exposure, so that the spectra of both objectsshould have the same telluric transmission spectrum. The telluric transmissionspectrum was constructed by dividing the observed spectrum of comparison starby its stellar template, and was used to correct the telluric absorption linesof the scientific object. Using the long slit spectrograph of Lijiang 2.4-metertelescope, we designed a long-term spectroscopic observation strategy, andfinished a four-year spectroscopic monitoring for a pair of objects (an activegalactic nuclei and an non-varying comparison star). We applied this method tocorrect the telluric absorption lines of the long-term monitored spectra byLijiang 2.4-meter telescope, and investigated the variation of the telluricabsorptions at Lijiang Observatory. We found that the telluric absorptiontransparency is mainly modulated by the seasonal variability of the relativehumidity, airmass and seeing. Using the scatter of the [O~III] $\lambda$5007fluxes emitted from the narrow-line region of active galactic nuclei as anindicator, we found that the correction accuracy of the telluric absorptionlines is 1%.

A correction method for the telluric absorptions and application to Lijiang Observatory