SMEs(Small　and　medium-sized　enterprises)　have　played　an　important　role　in　the　process　of　promoting　economic　and　social　development,　but　the　small　and　medium-sized　enterprises　are　often　at　a　disadvantage　in　the　process　of　trading　with　large　enterprises　because　of　their　own　conditions.　Through　using　the　dynamic　equations　of　duplicator,　giving　an　analysis　of　the　game　question　when　Small　and　medium-sized　suppliers　and　customers　signed　contracts　of　cash　on　delivery.　Educing　that　when　suppliers　and　customers　tend　to　co-operation,　more　contribute　to　the　development　of　enterprises　of　both　sides.　©　2010　IEEE.