An　innovative　method　is　created　for　transforming　iron-rich　RO　phase　(MgO0.239FeO0.761)　on　steel　slag　surface　into　nanostructured　Mg0.04Fe2.96O4　layer.　The　phase　change　process　is　investigated,　and　it　is　found　that　salicylic　acid　modification　and　alkaline　roasting　procedures　remarkably　increase　the　specific　surface　area　from　0.46　m2/g　(raw　steel　slag)　to　69.5　m2/g　(Mg0.04Fe2.96O4),　and　the　generation　of　Mg0.04Fe2.96O4　enhances　the　absorption　of　visible　light　and　Cr(VI)　conversion　with　2-times　increasement　than　raw　steel　slag.　Surface　complexation　between　H2C2O4　ligands　and　Fe　metal　moiety　on　Mg0.04Fe2.96O4　induces　the　intramolecular　electron　transfer　under　visible　light　irradiation　based　on　a　ligand-to-metal　charge　transfer　mechanism,　thus　resulting　in　Cr(VI)　photoreduction,　and　the　catalytic　efficiency　is　above　90%　for　Cr(VI)　(40　mg/L)　under　inherent　pH=　5.5　conditions.　Moreover,　recyclability　tests　based　on　magnetic　separation　show　that　the　photoreactivity　is　closely　related　to　Mg　content　of　Mg0.04Fe2.96O4　layer　where　Mg　leaching　occurs　and　finally　generates　cubic　spinel　configuration　Fe3O4.　This　work　highlights　the　importance　of　surface　functionalization　in　post-use　phases　of　steel　slag　in　which　surface　reactivity　and　application　potential　can　be　greatly　altered　by　chemical　exposure　history　and　surface　transformations.　It　also　provides　valuable　references　for　studying　the　metastable　state　mechanism　of　magnesium　ferrite　photocatalysts.