The relationship between spin fluctuations and superconductivity is an important issue in condensed matter physics. Part of evidence suggesting direct correlation between spin excitations and superconductivity is the evolution of low energy spin excitations in over-doped region. In La2-xSrxCuO4 superconductors, the evolution of spin excitations with doping in over-doped region provide a strong evidence. Similar results were also observed in BaFe2-xNixAs2. However, further measurements are still needed to determine the evolution and its correlation with superconductivty, especially on BaFe1.82Ni0.18As2 with Tc=8.7K while no spin resonance was observed in former experiments. We can establish a phase diagram of the effect of spin fluctuations on superconductivity through determining the evolution of low energy spin excitations in over-doped BaFe2-xNixAs.