  Topological insulator with antiferromagnetic order can serve as an ideal
platform for the realization of axion electrodynamics. In this paper, we report
a systematic study of the axion topological insulator candidate EuIn$_2$As$_2$.
A linear energy dispersion across the Fermi level confirms the existence of the
proposed hole-type Fermi pocket. Spin-flop transitions occur with magnetic
fields applied within the $ab$-plane while are absent for fields parallel to
the $c$-axis. Anisotropic magnetic phase diagrams are observed and the
orientation of the ground magnetic moment is found to be within the $ab$-plane.
The magnetoresistivity for EuIn$_2$As$_2$ behaves non-monotonic as a function
of field strength. It exhibits angular dependent evolving due to field-driven
and temperature-driven magnetic states. These results indicate that the
magnetic states of EuIn$_2$As$_2$ strongly affect the transport properties as
well as the topological nature.
