  Tsinghua-NAOC (National Astronomical Observatories of China) Telescope
(hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at
Xinglong bservatory of NAOC, with main scientific goals of monitoring various
transients in the universe such as supernovae, gamma-ray bursts, novae,
variable stars, and active galactic nuclei. We present in this paper a
systematic test and analysis of the photometric performance of this telescope.
Based on the calibration observations on twelve photometric nights, spanning
the period from year 2004 to year 2012, we derived an accurate transformation
relationship between the instrumental $ubvri$ magnitudes and standard Johnson
$UBV$ and Cousins $RI$ magnitudes. In particular, the color terms and the
extinction coefficients of different passbands are well determined. With these
data, we also obtained the limiting magnitudes and the photometric precision of
TNT. It is worthwhile to point out that the sky background at Xinglong
Observatory may become gradually worse over the period from year 2005 to year
2012 (e.g., $\sim$21.4 mag vs. $\sim$20.1 mag in the V band).
