  We have found a ferromagnetic Weyl semimetal (WSM) in half metallic
Co$_3$Sn$_2$Se$_2$. The three pairs of Weyl points near Fermi level (E$_F$) are
derived from nodal lines gapped by spin-orbit coupling (SOC). Though the Weyl
points are 0.11 eV above the charge neutral point, Fermi arc related states in
the cleaved surface can range from E$_F$ -0.15 to E$_F$ +0.11 eV in energy
space, due to the surface bands dispersion. Hence, Weyl points related physics
should be detected by surface measurements, such as ARPES and STM. Because of
the large Berry curvature deriving from the gapped nodal lines and Weyl points,
the anomalous Hall conductivity of Co$_3$Sn$_2$Se$_2$ can keep above 620 S/cm
in a large energy window. Beside magnetic WSM, Co$_3$Sn$_2$Se$_2$ also
possesses a Z$_2$ topological semimetal phase as the temperature is above Curie
temperature. Owing to the quasi two-dimensional lattice structure, it is also
possible to realize quantum anomalous Hall effect in its 2D limit via the size
effect. Therefore, Co$_3$Sn$_2$Se$_2$ provides a good platform for the
interplay of different topological states and magnetic states.
