  Based on the updated UrQMD transport model, the effect of the symmetry
potential energy on the two-nucleon HBT correlation is investigated with the
help of the coalescence program for constructing clusters, and the CRAB
analyzing program of the two-particle HBT correlation. An obvious non-linear
dependence of the neutron-proton (or neutron-neutron) HBT correlation function
($C_{np,nn}$) at small relative momenta on the stiffness factor $\gamma$ of the
symmetry potential energy is found: when $\gamma \lesssim 0.8$, the $C_{np,nn}$
increases rapidly with increasing $\gamma$, while it starts to saturate if
$\gamma \gtrsim 0.8$. It is also found that both the symmetry potential energy
at low densities and the conditions of constructing clusters at the late stage
of the whole process influence the two-nucleon HBT correlation with the same
power.
