  We report results on the searches of weakly interacting massive particles
(WIMPs) with sub-GeV masses ($m_{\chi}$) via WIMP-nucleus spin-independent
scattering with Migdal effect incorporated. Analysis on time-integrated (TI)
and annual modulation (AM) effects on CDEX-1B data are performed, with 737.1
kg$\cdot$day exposure and 160 eVee threshold for TI analysis, and 1107.5
kg$\cdot$day exposure and 250 eVee threshold for AM analysis. The sensitive
windows in $m_{\chi}$ are expanded by an order of magnitude to lower DM masses
with Migdal effect incorporated. New limits on $\sigma_{\chi N}^{\rm SI}$ at
90\% confidence level are derived as $2\times$10$^{-32}\sim7\times$10$^{-35}$
$\rm cm^2$ for TI analysis at $m_{\chi}\sim$ 50$-$180 MeV/$c^2$, and
$3\times$10$^{-32}\sim9\times$10$^{-38}$ $\rm cm^2$ for AM analysis at
$m_{\chi}\sim$75 MeV/$c^2-$3.0 GeV/$c^2$.
