  We study the effects of parity-time(PT)-symmetry on the photon blockade and
distinguish the different blockade mechanisms in a double-cavity optomechanical
system. By studying the light statistics of the system, we find the completely
different photon blockade behaviors when the PT-symmetry is broken or unbroken,
which is related to the PT phase transition. Furthermore, an interesting
phenomenon that the two cavities are blocked at the same time is found with the
appropriate system parameters. Those statistical phenomenons are all analyzed
in detail and demonstrated by analytically solving the Schrodinger equation and
numerically simulating the master equation, respectively. Finally, we also
consider the non-PT symmetric situations which further reveal the physical
essence of the photon blockade by comparing those results. Different from the
usual photon blockade, our proposal is feasible even with weak parameter
mechanism, i.e., the proposal neither requires the strong optomechanical
coupling nor the large tunneling coupling between cavities.
