  The effect of magnetic field on an ultrathin magnetic topological insulator
film with structural inversion asymmetry is investigated. We introduce the
phase diagram, calculate the Landau-level spectrum analytically and simulate
the transport behavior in Landauer-B\"uttiker formalism. The quantum anomalous
Hall phase will survive increasing magnetic field. Due to the two spin
polarized zero modes of Landau levels a nontrivial phase similar with quantum
spin Hall effect can be induced by magnetic field which is protected by
structural inversion symmetry. Some exotic longitudinal and Hall resistance
plateaus with fractional values are also found in a six terminal Hall bar,
arising from the coupling between edge states due to inverted energy band and
Landau levels.
