  By simultaneously describing the data of isospin sensitive nucleonic flow and
pion observables, such as $v_2^n/v_2^{ch}$ and $\pi^-/\pi^+$, with
ultra-relativistic quantum molecular dynamics (UrQMD) model, we got the
symmetry energy at flow and pion characteristic densities which are
$S(1.2\rho_0)=34\pm 4$ MeV and $S(1.5\rho_0)=36\pm 8$ MeV. Within the
uncertainties, the constraints of symmetry energy at characteristic densities
are consistent with the previous constraints by using other transport models.
The consistency suggests that the reliable constraints on symmetry energy
should be presented at the characteristic density of isospin sensitive
observables. By using the constraints of symmetry energy at two different
characteristic densities, the extrapolated value of $L$ is provided. Within
$2\sigma$ uncertainty, the extrapolated value of $L$ is in $5-70$ MeV which is
consistent with the recent combination analysis from PREX-II and astrophyiscs
data. Further, the calculations with the constrained parameter sets can
describe the data of charged pion multiplicities from S$\pi$RIT collaboration.
