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