  Topological flat band, on which the kinetic energy of topological electrons
is quenched, represents a platform for investigating the topological properties
of correlated systems. Recent experimental studies on flattened electronic
bands have mainly concentrated on 2-dimensional materials created by van der
Waals heterostructure-based engineering. Here, we report the observation of a
topological flat band formed by polar-distortion-assisted Rashba splitting in a
3-dimensional Dirac material ZrTe$_5$. The polar distortion and resulting
Rashba splitting on the band are directly detected by torque magnetometry and
the anomalous Hall effect, respectively. The local symmetry breaking further
flattens the band, on which we observe resistance oscillations beyond the
quantum limit. These oscillations follow the temperature dependence of the
Lifshitz-Kosevich formula but are evenly distributed in B instead of 1/B in
high magnetic fields. Furthermore, the cyclotron mass anomalously gets enhanced
about 10$^2$ times at field ~20 T. These anomalous properties of oscillations
originate from a topological flat band with quenched kinetic energy. The
topological flat band, realized by polar-distortion-assisted Rashba splitting
in the 3-dimensional Dirac system ZrTe$_5$, signifies an intrinsic platform
without invoking moir\'e or order-stacking engineering, and also opens the door
for studying topologically correlated phenomena beyond the dimensionality of
two.
