  We study numerically the Bloch electron wave-packet dynamics in periodic
potentials to simulate laser-solid interactions. We introduce a quasi-classical
model in the \emph{k} space combined with the energy band structure to
understand the high-order harmonic generation (HHG) process occurring in a
subcycle timescale. This model interprets the multiple plateau structure in HHG
spectra well and the linear dependence of cutoff energies on the amplitude of
vector potential of the laser fields. It also predicts the emission time of
HHG, which agrees well with the results by solving the time-dependent
Schr\"{o}dinger equation (TDSE). It provides a scheme to reconstruct the energy
dispersion relations in Brillouin zone and to control the trajectories of HHG
by varying the shape of laser pulses. This model is instructive for
experimental measurements.
