C.B.　Roellgen　polymorphic　cipher　(PMC)　has　been　found　wide　application　in　encrypting　diskette,　but　it　is　not　suitable　for　complicated　network　environments.　For　this　reason,　PMC　was　improved　and　a　new　construction　method　of　S-box　was　proposed　based　on　a　strong　one-way　function　which　could　satisfy　the　Strict　Avalanche　Criterion　(SAC).　It　could　create　function　which　could　satisfy　SAC　quickly　and　conveniently.　Finally,　the　security　of　the　method　was　analyzed　and　verified　in　terms　of　SAC.　The　experimental　results　show　that　many　long　periodic　sequences　which　can　satisfy　cryptography　characteristics　can　be　produced　quickly　and　largely　by　the　improved　PMC　and　used　as　the　session　keys　to　both　communication　parties　in　network.