  The optical selection rules of an electron system under a magnetic field play
key roles in determining its optical properties, from which the band structures
and underlying symmetries can be derived. In this Letter, based on a
three-dimensional strong topological insulator model describing ZrTe5,we study
the Landau levels (LLs) and magneto-optical conductivity under an in-plane
magnetic field. We reveal that in the transverse conductivity Re(\sigma_{zz}),
the unconventional optical selection rules n\righatarrow n\pm 2 dominate, with
n being the LL index. We attribute the unconventional selection rules to the
peculiar distribution of parity carried by the LLs, resulting from the chiral
symmetry of the sub-Hamiltonians. Moreover, we predict that, if the strong
anisotropic system is tuned to be nearly isotropic, the LLs would redistribute
and the conventional selection rules n\rightarrow n\pm 1 can be recovered.
