In　order　to　solve　the　problem　of　protection　and　sharing　of　information　in　information　system　with　multilevel　security,　the　security　requirements　of　information　systems　were　analyzed,　the　BLP　model　of　stand-alone　computer　systems　was　extended　to　information　systems,　the　new　interpretations　of　subjects　and　objects　in　information　systems　was　introduced,　the　new　need-to-share　category　was　added　to　subjects　and　objects,　and　a　multilevel　security　model　for　information　sharing　was　developed.　Then,　the　formal　description　of　the　security　model　was　presented,　the　full　set　of　access　rules　that　apply　to　the　system　were　established,　and　the　security　of　the　information　system　was　proved.　The　new　security　model　allows　information　being　securely　shared　with　the　right　users　and　protected　from　the　wrong　user,　while　maintaining　the　multilevel　security　of　information　system.