  Twisted bilayer graphene (TBG) can host the moir\'{e} energy flat bands with
two-fold degeneracy serving as a fruitful playground for strong correlations
and topological phases. However, the number of degeneracy is not limited to
two. Introducing a spatially alternative magnetic field, we report that the
induced magnetic phase becomes an additional controllable parameter and leads
to an undiscovered generation of four-fold degenerate flat bands. This
emergence stems from the band inversion at $\Gamma$ point near the Fermi level
with a variation of both twisted angle and magnetic phase. We present the
conditions for the emergence of multi-fold degenerate flat bands, which are
associated with the eigenvalue degeneracy of a Birman-Schwinger operator. Using
holomorphic functions, which explain the origin of the double flat bands in the
conventional TBG, we can generate analytical wave functions in the magnetic TBG
to show absolute flatness with four-fold degeneracy. Moreover, we identify an
orbital-related intervalley coherent state as the many-body ground state at
charge neutrality. In contrast, the conventional TBG has only two moir\'{e}
energy flat bands, and the highly degenerate flat bands with additional orbital
channels in this magnetic platform might bring richer correlation physics.
