  Recently, the magnetic domain walls have been experimentally observed in
antiferromagnetic topological insulators MnBi$_2$Te$_4$, where we find that the
topological zero-line modes (ZLMs) appear along the domain walls. Here, we
theoretically demonstrate that these ZLMs are layer-dependent in MnBi$_2$Te$_4$
multilayers. For domain walls with out-of-plane ferromagnetism, we find that
ZLMs are equally distributed in the odd-number layers. When domain walls
possess in-plane magnetization, the ZLMs can also exist in even-number layers
due to in-plane mirror-symmetry breaking. Moreover, the conductive channels are
mainly distributed in the outermost layers with increasing layer thickness. Our
findings lay out a strategy in manipulating ZLMs and also can be utilized to
distinguish the corresponding magnetic structures.
