  The real-time dynamics of chiral phase transition is investigated in a
two-flavor ($N_f=2$) soft-wall AdS/QCD model. To understand the dynamics of
thermalization, we quench the system from initial states deviating from the
equilibrium states. Then, we solve the nonequilibrium evolution of the order
parameter (chiral condensate $\langle \sigma\equiv\bar{q}q\rangle$). It is
shown that the system undergoes an exponential relaxation at temperatures away
from the critical temperature $T_c$. The relaxation time diverges at $T_c$,
presenting a typical behavior of critical slowing down. Numerically, we extract
the dynamic critical exponent $z$, and get $z\approx 2$ by fitting the scaling
behavior $\sigma\propto t^{-\beta/(\nu z)}$, where the mean-field static
critical exponents (order parameter critical exponent $\beta=1/2$, correlation
length critical exponent $\nu=1/2$ ) have been applied. More interestingly, it
is remarked that, for a large class of initial states, the system would linger
over a quasi-steady state for a certain period of time before the
thermalization. It is suggested that the interesting phenomenon, known as
prethermalization, has been observed in the framework of holographic models. In
such prethermal stage, we verify that the system is characterized by a
universal dynamical scaling law and described by the initial-slip exponent
$\theta=0$.
