  We present a new method of calculating distances of molecular clouds in the
Galactic plane, using CO observations and the Gaia DR2 parallax and G-band
extinction ($A_G$) measurements. Due to the complexity of dust environments in
the Galactic plane, $A_G$ contains irregular variations, which is difficult to
model. To overcome this difficulty, we propose that the $A_G$ of off-cloud
stars (Gaia stars around molecular clouds) can be used as a baseline to
calibrate the $A_G$ of on-cloud stars (Gaia stars toward molecular clouds),
which removes the $A_G$ components that are unrelated to molecular clouds. The
distance is subsequently inferred from the jump point in on-cloud $A_G$ with
Bayesian analysis and Markov chain Monte Carlo (MCMC) sampling. We applied this
baseline subtraction method to a 100 square degree region (209.75{\deg} $\leq$
l $\leq$ 219.75{\deg} and |b| $\leq$ 5{\deg}) in the third Galactic quadrant,
which was mapped as part of the Milky Way Imaging Scroll Painting (MWISP)
project, covering three CO isotopologue lines, and derived distances and masses
for 11 molecular clouds, including the Maddalena molecular cloud and Sh 2-287.
The results indicate that the distance of the Perseus Arm in this region is
about 2.4 kpc and molecular clouds are present in the interarm regions.
