  In a strong magnetic field, a high-energy photon can be absorbed and then
produce an electron-positron pair. The produced electron/positron will in turn
radiate a high-energy photon via synchrotron radiation, which then initiates a
cascade. We built a one-dimensional Monte-Carlo code to study the development
of the cascade especially after it reaches the saturated status, when almost
all the energy of the primary particles transfers to the photons. The photon
spectrum in this status has a cut-off due to the absorption by magnetic fields,
which is much sharper than the exponential one. Below the cut-off, the spectral
energy distribution (SED) manifest itself as a broken power-law with a spectral
index of $0.5$ and $0.125$, respectively, below and above the broken energy.
The SED can be fitted by a simple analytical function, which is solely
determined by the product of the cascade scale $R$ and the magnetic field
perpendicular to the motion of the particle B_{\perp}, with an accuracy better
than 96\%. The similarity of the spectrum to that from the cascade in an
isotropic black-body photon field is also studied.
