Freeway　should　be,　and　has　been　the　safest　highway　compared　to　other　types　of　highway　due　to　its　highest　design　standard.　These　years　Chinese　government　was　forced　to　pay　more　attention　to　freeway　safety　problems　due　to　its　leading　fatality　rates　despite　its　luxury　infrastructure　criteria.　Research　on　crash　rates　and　speed　has　been　achieved　by　many　people,　but　paper　about　the　relationship　between　crash　rates　and　average　speed　difference　between　cars　and　large　vehicles　on　freeway　kept　rare.　This　research　explored　this　relationship　using　three-year　crash　data　and　traffic　flow　data　of　one　freeway　of　Beijing　suburban　section.　From　the　result　it　can　be　concluded　that　speed　difference　and　crash　rates　are　correlative,　and　similar　empirical　functions　have　been　obtained　using　compound,　growth,　and　exponential　curve　to　fit.　The　general　law　extracted　from　this　paper　shows　that　with　the　increasing　of　the　average　speed　difference,　the　crash　rates　increase　rapidly.