  MnBi$_2$Te$_4$ is an antiferromagnetic topological insulator which stimulates
intense interests due to the exotic quantum phenomena and promising device
applications. Surface structure is a determinant factor to understand the novel
magnetic and topological behavior of MnBi2Te4, yet its precise atomic structure
remains elusive. Here, we discovered a spontaneous surface collapse and
reconstruction in few-layer MnBi2Te4 exfoliated under delicate protection.
Instead of the ideal septuple-layer structure in the bulk, the collapsed
surface is shown to reconstruct as Mn-doped Bi$_2$Te$_3$ quintuple-layer and
Mn$_x$Bi$_y$Te double-layer with a clear van der Waals gap in between.
Combining with first-principles calculations, such spontaneous surface collapse
is attributed to the abundant intrinsic Mn-Bi antisite defects and tellurium
vacancy in the exfoliated surface, which is further supported by in-situ
annealing and electron irradiation experiments. Our results shed light on the
understanding of the intricate surface-bulk correspondence of MnBi$_2$Te$_4$,
and provide insightful perspective of the surface-related quantum measurements
in MnBi$_2$Te$_4$ few-layer devices.
