Overdoped BaFe1.2Ru0.8As2 is an ideal system for comparison study between band structure calculation and spin excitation spectrum, because the electron correlation is expected to be weakened by Ru substitution due to the large spatial extension of the Ru 4d orbitals. The ARPES measurement shows that the isovalent Ru doping in BaFe2As2 reduces the electron correlations significantly, especially in the overdoped regime. Here we propose to use inelastic neutron scattering to study the magnetic excitations in the weakly correlated overdoped superconductor BaFe1.2Ru0.8As2 (Tc=10 K). Comparing the spin excitation dispersions with the band structures in this system will help to elucidate the long debated issue: whether local moment or itinerant picture is more appropriate for iron based superconductors.