  We present the ultraviolet magnitudes for over three million stars in the
LAMOST survey, in which 2,202,116 stars are detected by $GALEX$. For 889,235
undetected stars, we develop a method to estimate their upper limit magnitudes.
The distribution of (FUV $-$ NUV) shows that the color declines with increasing
effective temperature for stars hotter than 7000 K in our sample, while the
trend disappears for the cooler stars due to upper atmosphere emission from the
regions higher than their photospheres. For stars with valid stellar
parameters, we calculate the UV excesses with synthetic model spectra, and find
that the (FUV $-$ NUV) vs. $R'_{\mathrm{FUV}}$ can be fitted with a linear
relation and late-type dwarfs tend to have high UV excesses. There are 87,178
and 1,498,103 stars detected more than once in the visit exposures of $GALEX$
in the FUV and NUV, respectively. We make use of the quantified photometric
errors to determine statistical properties of the UV variation, including
intrinsic variability and the structure function on the timescale of days. The
overall occurrence of possible false positives is below 1.3\% in our sample. UV
absolute magnitudes are calculated for stars with valid parallaxes, which could
serve as a possible reference frame in the NUV. We conclude that the colors
related to UV provide good criteria to distinguish between M giants and M
dwarfs, and the variability of RR Lyrae stars in our sample is stronger than
that of other A and F stars.
