  Within the framework of semiclassical Boltzmann-Uehling-Uhlenbeck (BUU)
transport model, we investigated the effects of symmetry energy on the
sub-threshold pion using the isospin MDI interaction with the stiff and soft
symmetry energies in the central collision of $^{48}$Ca + $^{48}$Ca at the
incident beam energies of 100, 150, 200, 250 and 300 MeV/nucleon, respectively.
We find that the ratio of $\pi^{-}/\pi^{+}$ of sub-threshold charged pion
production is greatly sensitive to the symmetry energy, particularly around 100
MeV/nucleon energies. Large sensitivity of sub-threshold charged pion
production to nuclear symmetry energy may reduce uncertainties of probing
nuclear symmetry energy via heavy-ion collision.
