  By using the functional path integral method, we obtain a model-independent
formula for nuclear symmetry energy, which explicitly shows the relation
between nuclear symmetry energy and isospin susceptibility. The latter one is
found to be a probe to the QCD chiral phase transition. We further found that,
the nuclear symmetry energy has an abrupt change at the critical nuclear
density where the chiral symmetry restores partially, which could be detected
from the experiments.
