  The neutron skin thickness of the benchmark nucleus $^{208}$Pb is crucial for
our understanding of the equation of state of nuclear matter. In this paper, we
discuss the effect of the neutron skin on the flow ratio observables in the
semi-isobaric collisions $^{208}$Pb+$^{208}$Pb and $^{197}$Au+$^{197}$Au using
iEBE-VISHNU hydrodynamic simulations. Our results suggest that $^{208}$Pb and
$^{197}$Au should have the same magnitude of neutron skin thickness to describe
the anisotropic flow ratios between the semi-isobaric systems. Our method
provides an unconventional way to determine the neutron skin with the existing
relativistic heavy ion collision data.
