  The parity-time ($\mathcal{PT}$) symmetric structures have exhibited
potential applications in developing various robust quantum devices. In an
optical trimmer with balanced loss and gain, we analytically study the
$\mathcal{PT}$ symmetric phase transition by investigating the spontaneous
symmetric breaking. We also illustrate the single-photon transmission behaviors
in both of the $\mathcal{PT}$ symmetric and $\mathcal{PT}$ symmetry broken
phases. We find (i) the non-periodical dynamics of single-photon transmission
in the $\mathcal{PT}$ symmetry broken phase instead of $\mathcal{PT}$ symmetric
phase can be regarded as a signature of phase transition; and (ii) it shows
unidirectional single-photon transmission behavior in both of the phases but
comes from different underlying physical mechanisms. The obtained results may
be useful to implement the photonic devices based on coupled-cavity system.
