There　is　a　hidden　danger　with　the　shortage　of　acceleration　lane　length　at　freeway　interchange.　In　order　to　drive　safely,　this　paper　consults　the　traditional　method　of　calculating　the　length　of　acceleration　lane,　and　limits　the　scope　of　each　calculation　parameter.　Base　on　above　calculations,　an　analysis　of　the　effect　of　road　and　traffic　conditions　to　acceleration　lane　length　is　made.　A　new　formula　is　presented　in　order　to　adjust　the　length　in　theory　to　meet　the　demand　in　practice　through　the　amended　vertical　slope　and　traffic　volume　of　main　lanes.