  We investigate the role of disorder in the edge transport of axion insulator
films. We predict by first-principles calculations that even-number-layer
MnBi$_2$Te$_4$ have gapped helical edge states. The random potential will
dramatically modify the edge spectral function to become gapless. However, such
gapless helical state here is fundamentally different from that in quantum spin
Hall insulator or topological Anderson insulator. We further study the edge
transport in this system by Landauer-B\"{u}ttiker formalism, and find such
gapless edge state is dissipative and not immune to backscattering, which would
explain the dissipative nonlocal transport in the axion insulator state
observed in six septuple layer MnBi$_2$Te$_4$ experimentally. Several transport
experiments are proposed to verify our theory on the dissipative helical edge
channels. In particular, the longitudinal resistance can be greatly reduced by
adding an extra floating probe even if it is not used. These results will
facilitate the observsation of long-sought topological magnetoelectric effect
in axion insulators.
