  We theoretically investigate the spectral caustics of high-order harmonics in
solids. We analyze the 1-dimension model of solids HHG and find that, apart
from the caustics originated from the van Hove singularities in the energy-band
structure, another kind of catastrophe singularities also emerge when the
different branches of electron-hole trajectories generating high-order
harmonics coalesce into a single branch. We solve time-dependent Schr\"odinger
equation in periodic potential and demonstrate the control of this kind of
singularities in HHG with the aids of two-color laser fields. The diffraction
patterns of the harmonic spectrum near the caustics agree well with the
inter-band electron-hole recombination trajectories predicted by the
semiconductor semi-classical equation. This work is expected to help to
understand the HHG dynamics in solids and manipulate the harmonic spectrum by
adjusting driving field parameters.
