  We demonstrate a $\mathcal{PT}$-symmetry-breaking chaos in optomechanical
system (OMS), which features an ultralow driving threshold. In principle, this
chaos will emerge once a driving laser is applied to the cavity mode and lasts
for a period of time. The driving strength is inversely proportional to the
starting time of chaos. This originally comes from the dynamical enhancement of
nonlinearity by field localization in $\mathcal{PT}$-symmetry-breaking phase
($\mathcal{PT}$BP). Moreover, this chaos is switchable by tuning the system
parameters so that a $\mathcal{PT}$-symmetry phase transition occurs. This work
may fundamentally broaden the regimes of cavity optomechanics and nonlinear
optics. It offers the prospect of exploring ultralow-power-laser triggered
chaos and its potential applications in secret communication.
