  We present spectroscopic measurements on the night sky of Xinglong
Observatory for a period of 12 years from 2004 to 2015. The spectra were
obtained on moonless clear nights using the OMR spectrograph mounted on a
2.16-m reflector with a wavelength coverage of 4000-7000A. The night sky
spectrum shows the presence of emission lines from Hg I and Na I due to local
artificial sources, along with the atmospheric emission lines, i.e., O I and OH
molecules, indicating the existence of light pollution. We have monitored the
night sky brightness during the whole night and found some decrement in the sky
brightness with time, but the change is not significant. Also, we monitored the
light pollution level in different azimuthal directions and found that the
influence of light pollution from the direction of Beijing is stronger compared
with that from the direction of Tangshan and other areas. An analysis of night
sky spectra for the entire data set suggested that the zenith sky brightness of
Xinglong Observatory has brightened by about 0.5 mag arcsec -2 in the V and B
bands from 2004 to 2015. We recommend consecutive spectroscopic measurements of
the night sky brightness at Xinglong Observatory in the future, not only for
monitoring but also for scientific reference.
