Cascading　failure　in　power　grid　is　a　key　factor　in　large　power　blackouts.　Through　analyzing　historical　power　blackouts,　it　is　found　that　relay　protection　plays　an　important　role　in　cascading　failure　propagation.　However,　existing　cascading　failure　model　cannot　accurately　reflect　the　relay　protection　action.　In　this　paper,　a　model　integrating　relay　protection　for　cascading　failure　based　on　AC　power　flow　is　proposed.　In　the　model,　relay　protection　systems　including　distance　protection,　pilot　distance　protection　and　cumulative　overloading　protection　are　described　and　integrated　respectively.　According　to　the　proposed　model,　a　simulation　algorithm　of　cascading　failure　process　is　also　developed.　In　the　experiment,　our　model　is　compared　with　the　DC　power　flow　model　and　topological　model.　The　experimental　results　show　that　the　probability　of　demand　loss　in　our　model　is　significantly　lower　than　the　other　two　models,　which　is　consistent　with　the　dynamic　behavior　of　entire　power　system　in　the　actual　power　grid.　©　2020　IEEE.