  We develop a model to describe the mixed valence regime in magic-angle
twisted bilayer graphene (MATBG) using the recently developed heavy-fermion
framework. By employing the large-$N$ slave-boson approach, we derive the
self-consistent mean field equations and solve them numerically. We find that
the SU(8) symmetry constraint moir\'e system exhibits novel mixed-valence
properties which are different from conventional heavy-fermions systems. We
find the solutions describing the physics at the filling near the Mott
insulator regime in the limit of strong Coulomb interactions between the
flat-band fermions. Our model can provide additional insight into the possible
microscopic origin of unconventional superconductivity in MATBG.
