The recent progress in the experimental measurement of charged pions in HIC at 1 GeV/u energy region is reviewed.Based on the 1BUU04 simulation, a soft symmetry energy at density near $2\rho$ is needed to account for the experimental data.The sensitivity of probing $E_{sym} (\rho)$ in HIC via charged pion ratio is studied by varying the system size and beam energy to suggest the optimized conditions for further experimental measurements.