  We theoretically study the dynamics of typical optomechanical systems,
consisting of a passive optical mode and an active mechanical mode, in the
$\mathcal{PT}$- and broken-$\mathcal{PT}$-symmetric regimes. By fully
analytical treatments for the dynamics of the average displacement and particle
numbers, we reveal the phase diagram under different conditions and the various
regimes of both $\mathcal{PT}$-symmetry and stability of the system. We find
that by appropriately tuning either mechanical gain or optomechanical coupling,
both phase transitions of the $\mathcal{PT}$-symmetry and stability of the
system can be flexibly controlled. As a result, the dynamical behaviors of the
average displacement, photons, and phonons are radically changed in different
regimes. Our study shows that $\mathcal{PT}$-symmetric optomechanical devices
can serve as a powerful tool for the manipulation of mechanical motion,
photons, and phonons.
