With　the　rapid　development　of　informationization　process,　the　fundamental,　global　and　strategic　role　of　critical　information　systems　is　increasingly　significant.　For　addressing　the　access　control　problem　of　information　systems　with　multilevel　security,　the　BLP　model　of　stand-alone　computer　systems　is　extended　to　information　systems,　the　new　interpretations　of　subjects　and　objects　in　the　information　systems　are　presented,　the　security　level　ranges　of　subject　and　object　are　introduced,　and　a　security　model　for　information　systems　with　multi-level　security　is　proposed.　Then,　the　formal　description　of　the　security　model　is　included,　the　full　set　of　access　rules　is　established,　and　the　security　of　the　model　is　proved.　The　new　security　model　maintains　the　multi-level　security　of　information　systems,　while　allows　security　level　ranges　of　subject　and　object　being　dynamically　changed.　©　2011　IEEE.