  The generalized tight-binding model, being based on the spin-dependent
sublattices, is developed to explore the magnetic quantization of monolayer
bismuthene. The sp$^{3}$ orbital hybridizations, site energies, nearest and
next-nearest hopping integrals, spin-orbital interactions and magnetic field
(${B_{z}}$ ${\hat{z}}$) are taken into account simultaneously. There exist
three groups of low-lying Landau levels (LLs), in which they are mainly from
the (6p$_{x}$,6p$_{y}$,6p$_{z}$) orbitals, and only the first group belongs to
the unoccupied conduction states. Furthermore, each group is further split into
the spin-up- and spin-down-dominated subgroups. The six subgroups present the
rich and unique $B_{z}$-dependent LL energy spectra, covering the specific or
arc-shaped $% B_{z}$-dependences, the normal/irregular spin-split energies, and
the non-crossing/crossing/anti-crossing behaviors. Specially, the second group
of valence LLs near the Fermi level can create the frequent inter-subgroup LL
anti-crossings since the main and side modes are comparable. The main features
of energy spectra can create the special structures in density of states.
