An　access　control　points　in　cloud　computing　are　difficult　to　link.　An　assurance　model　for　access　control　on　the　whole　system　was　developed　based　on　formal　definitions　of　the　access　request　equivalence　relation　and　the　support　relation,　the　analysis　formally　proves　that　the　assurance　algorithm　can　ensure　the　credibility　of　access　requests.　The　implementation　methods　are　given　for　the　network　layer,　application　layer　and　operating　system　kernel　layer　in　cloud　computing.　An　access　semantic　encapsulation　shows　that　the　algorithm　meets　the　access　control　linkage　requirements　and　can　ensure　the　credibility　of　access　requests.　©　2017,　Tsinghua　University　Press.　All　right　reserved.