  We study the energy spectrum of moir\'e systems under a uniform magnetic
field. The superlattice potential generally broadens Landau levels into Chern
bands with finite bandwidth. However, we find that these Chern bands become
flat at a discrete set of magnetic fields which we dub "magic zeros". The flat
band subspace is generally different from the Landau level subspace in the
absence of the moir\'e superlattice. By developing a semiclassical quantization
method and taking account of superlattice induced Bragg reflection, we prove
that magic zeros arise from the simultaneous quantization of two distinct
$k$-space orbits. The flat bands at magic zeros provide a new setting for
exploring crystalline fractional quantum Hall physics.
