Controllable　anionic　redox　for　a　transformational　increase　in　the　energy　density　is　the　pursuit　of　next　generation　Li-ion　battery　cathode　materials.　Its　activation　mechanism　is　coupled　with　the　local　coordination　environment　around　O,　which　posts　experimental　challenges　for　control.　Here,　the　tuning　capability　of　anionic　redox　is　shown　by　varying　O　local　environment　via　experimentally　controlling　the　density　of　stacking　faults　in　Li2MnO3,　the　parent　compound　of　Li-rich　oxides.　By　combining　computational　analysis　and　spectroscopic　study,　it　is　quantitatively　revealed　that　more　stacking　faults　can　trigger　smaller　Li-O-Li　bond　angles　and　larger　Li-O　bond　distance　in　local　Li-rich　environments　and　subsequently　activate　oxygen　redox　reactivity,　which　in　turn　enhances　the　reactivity　of　Mn　upon　the　following　reduction　process.　This　study　highlights　the　critical　role　of　local　structure　environment　in　tuning　the　anionic　reactivity,　which　provides　guidance　in　designing　high-capacity　layered　cathodes　by　appropriately　adjusting　stacking　faults.