  Long-term spectroscopic monitoring campaigns on active galactic nuclei (AGNs)
provide a wealth of information about its interior structure and kinematics.
However, a number of the observations suffer from the contamination of
second-order spectra (SOS) which will introduce some undesirable uncertainties
at the red side of the spectra. In this paper, we test the effect of SOS and
propose a method to correct it in the time domain spectroscopic data using the
simultaneously observed comparison stars. Based on the reverberation mapping
(RM) data of NGC 5548 in 2019, one of the most intensively monitored AGNs by
the Lijiang 2.4 m telescope, we find that the scientific object, comparison
star, and spectrophotometric standard star can jointly introduce up to similar
to 30% SOS for Grism 14. This irregular but smooth SOS significantly affects
the flux density and profile of the emission line, while having little effect
on the light curve. After applying our method to each spectrum, we find that
the SOS can be corrected effectively. The deviation between corrected and
intrinsic spectra is similar to 2%, and the impact of SOS on time lag is very
minor. This method makes it possible to obtain the H alpha RM measurements from
archival data provided that the spectral shape of the AGN under investigation
does not have a large change.
