  We perform a comprehensive CO study toward the Monoceros OB1 (Mon OB1) region
based on the MWISP survey at an angular resolution of about $50''$. The
high-sensitivity data, together with the high dynamic range, shows that
molecular gas in the $\rm 8^{\circ}\times4^{\circ}$ region displays complicated
hierarchical structures and various morphology (e.g., filamentary, cavity-like,
shell-like, and other irregular structures). Based on Gaussian decomposition
and clustering for $\mathrm{^{13}CO}$ data, a total of 263 $\mathrm{^{13}CO}$
structures are identified in the whole region, and 88% of raw data flux is
recovered. The dense gas with relatively high column density from the
integrated CO emission is mainly concentrated in the region where multiple $\rm
^{13}CO$ structures are overlapped. Combining the results of 32 large
$\mathrm{^{13}CO}$ structures with distances from Gaia DR3, we estimate an
average distance of $\rm 729^{+45}_{-45}~pc$ for the GMC complex. The total
mass of the GMC Complex traced by $\mathrm{^{12}CO}$, $\mathrm{^{13}CO}$, and
$\mathrm{C^{18}O}$ are $1.1\times10^5~M_\odot$, $4.3\times10^4~M_\odot$, and
$8.4\times10^3~M_\odot$, respectively. The dense gas fraction shows a clear
difference between Mon OB1 GMC East (12.4%) and Mon OB1 GMC West (3.3%). Our
results show that the dense gas environment is closely linked to the nearby
star-forming regions. On the other hand, star-forming activities have a great
influence on the physical properties of the surrounding molecular gas (e.g.,
greater velocity dispersion, higher temperatures, and more complex velocity
structures, etc.). We also discuss the distribution/kinematics of molecular gas
associated with nearby star-forming activities.
