  The $\pi^-/\pi^+$ ratio is examined as a function of isospin asymmetry and
beam energy for central collisions induced by neutron-rich nuclei within an
isospin-dependent hadronic transport model. The $\pi^-/\pi^+$ ratio is found to
increase with the isospin asymmetry, but decrease with the beam energy of the
reaction. For neutron-rich systems, such as, $^{124}Sn+^{124}Sn$ and
$^{132}Sn+^{124}Sn$, the sensitivity of $\pi^-/\pi^+$ ratio to nuclear symmetry
energy is almost beam energy independent.
