  We construct a continuum model for the Moir\'e superlattice of twisted
bilayer MnBi$_2$Te$_4$, and study the band structure of the bilayer in both
ferromagnetic (FM) and antiferromagnetic (AFM) phases. We find the system
exhibits highly tunable Chern bands with Chern number up to $3$. We show that a
twist angle of $1^\circ$ turns the highest valence band into a flat band with
Chern number $\pm1$ that is isolated from all other bands in both FM and AFM
phases. This result provides a promising platform for realizing time-reversal
breaking correlated topological phases, such as fractional Chern insulator and
$p+ip$ topological superconductor. In addition, our calculation indicates that
the twisted stacking facilitates the emergence of quantum anomalous Hall effect
in MnBi$_2$Te$_4$.
