Li-rich　layered　oxides　(LLOs),　contributing　ultrahigh　discharge　capacity　and　energy　density,　are　considered　as　promising　cathode　candidates　for　Li-ion　batteries.　However,　thermodynamically　spontaneous　transition　metal　(TM)　migration　and　irreversible　oxygen　loss,　which　are　originated　from　the　characteristic　LiMn6　ordering　domains　in　the　TM　layer,　usually　lead　to　rapid　capacity　decline,　severe　voltage　fading　and　voltage　hysteresis　of　LLOs.　Herein,　distinctive　MgMn6　superstructure　is　incorporated　into　the　TM　layers　through　a　unique　anti-site　Mg2+　doping　strategy　to　disperse　the　original　aggregated　LiMn6　superstructure　and　consequently　solve　the　above　issues　of　LLOs.　Specifically,　Mg2+　are　pre-incorporated　into　the　TM　layers　of　P3-type　NaTMO2,　which　is　subsequently　transformed　to　O3-LLOs　via　an　ion-exchange　method,　instead　of　the　traditional　sintering　process　usually　leading　to　Mg2+　incorporation　into　the　Li　layers.　Series　of　in-situ/ex-situ　characterizations　and　theoretical　calculations　reveal　that,　owing　to　the　pinning　effect　of　the　incorporated　MgMn6　superstructure,　the　anti-site　Mg2+　doping　can　inherently　retard　the　in-plane/out-plane　TM　migration　and　improve　the　anionic　reversibility,　leading　to　the　enhanced　stability　of　the　layered　structure　upon　cycling.　Therefore,　the　anti-site　Mg　doped　Li0.7[Li0.18Mg0.02Mn0.65Co0.15]O2　cathode　exhibits　an　excellent　capacity　retention　of　86.22　%　and　mitigated　voltage　fading　of　0.40　mV　per　cycle　after　500　cycles　at　1C.　These　findings　provide　inspiring　insights　for　superstructure　design　in　Li-rich　Mn-based　cathode　materials,　and　open　up　new　path　to　develop　advanced　cathodes　materials　with　reversible　anionic　redox　chemistry.