  The impact parameter dependence of \rpi ratio is examined in heavy-ion
collisions at 400MeV/nucleon within a transport model. It is shown that the
sensitivity of \rpi ratio on symmetry energy shows a transition from central to
peripheral collisions, i.e., the stiffer symmetry energy leads to a larger \rpi
ratio in peripheral collisions while the softer symmetry energy always leads
this ratio to be larger in central collisions. After checking the kinematic
energy distribution of \rpi ratio, we found this transition of sensitivity of
\rpi ratio to symmetry energy is mainly from less energetic pions, i.e., the
softer symmetry energy gets the less energetic pions to form a smaller \rpi
ratio in peripheral collisions while these pions generate a larger \rpi ratio
in central collisions. Undoubtedly, the softer symmetry energy can also lead
more energetic pions to form a larger \rpi ratio in peripheral collisions.
Nevertheless, considering that most of pions are insufficient energetic at this
beam energy, we therefore suggest the \rpi ratio as a probe of the high-density
symmetry energy effective only in central at most to midcentral collisions,
thereby avoiding the possible information of low-density symmetry energy
carried in \rpi ratio from peripheral collisions.
