The possibility of the experiment for constraining the symmetry energy Esym(ρ)at supra-densities via π-/π+probe on the extal target experiment of phase Ⅰ(ETE(Ⅰ)with part coverage at forward angle at HIRFL-CSR is studied for the first time by using the isospin and momentum dependent hadronic transport model IBUU04.Based on the transport simulation with Au+Au collisions at 400 MeV/u,it is found that the differential π-/π+ratios are more sensitive to Esym(p)at forward angles in laboratory reference,compared with the total yield ratio widely proposed.The insufficient coverage at lower transverse momentum maintains the sensitivity of the dependence of π-/π+ratio on the Esym(ρ)at high density,indicating that the ETF(Ⅰ)under construction in Lanzhou provides the possibility of performing the experiment for probing the asymmetric nuclear equation of state.