  We theoretically investigate the stability of a two cascaded cavity
optomechanical system with optical parametric amplifiers (OPAs) inside the two
coupled cavities, and study the steady-state entanglement between two distant
mechanical resonators. We show that the parameter regime where the system is
unstable without OPAs, such as relatively high laser intensity and blue
detuning, can be exploited to build the steady-state mechanical entanglement by
modulating the parametric gain. The application of OPAs is helpful to preserve
the mechanical entanglement suffered from the dissipation at some finite
temperature. The scheme provides an alternative way for improving and
engineering the quantum entanglement of two distant mechanical oscillators.
