  How to realize applicably appreciated functionalities based on the coupling
between charge and spin degrees of freedom is still a challenge in the field of
spintronics. For example, anisotropic magnetoresistance (AMR) effect is
utilized to read out the information stored by various magnetic structures,
which usually originates from atomic spin-orbit coupling (SOC). However, the
application of AMR in antiferromagnet-based spintronics is still hindered by
rather small AMR value. Here, we discover a colossal AMR effect during the
field-induced metal-to-insulator transition (MIT) in a nearly Dirac material
EuMnSb$_2$ with an antiferromagnetic order of Eu$^{2+}$ moments. The colossal
AMR reaches to an unprecedented value of 1.84$\times$10$^6$% at 2 K, which is
four orders of magnitude larger than previously reported values in
antiferromagnets. Based on density functional theory calculations, a Dirac-like
band structure, which is strongly dependent on SOC, is confirmed around Y point
and dominates the overall transport properties in the present sample with
predominant electron-type carriers. Moreover, it is also revealed that the
indirect band gap around Fermi level is dependent on the magnetic structure of
Eu$^{2+}$ moments, which leads to the field-induced MIT and plays a key role on
the colossal AMR effect. Finally, our present work suggests that the similar
antiferromagnetic topological materials as EuMnSb$_2$, in which Dirac-like
fermions is strongly modulated by SOC and antiferromagnetism, would be a
fertile ground to explore applicably appreciated AMR effect.
