It　is　often　meet　in　the　process　of　designing　the　expressway　in　the　mountainous　area　to　design　long　steep　downgrade　section.　But　the　current　route　design　specification　doesn＇t　have　the　definition　criterion　of　long　steep　downgrade　sections.　Starting　from　the　driver＇s　driving　behavior　and　driving　needs,　the　running　speed　was　taken　as　the　macro　performance　of　driving　behavior　to　analyze　the　performance　of　driving　behavior　in　this　article.　Some　experimental　data　were　collected　from　the　real　vehicle　test　on　20　truck　drivers　in　the　mountainous　expressway　long　steep　downgrade　section.　By　analyzing　the　data,　the　correlation　between　the　speed　with　the　average　longitudinal　slope　and　the　longitudinal　slope　length　was　found,　and　a　prediction　model　was　established　for　the　truck　operating　speed.　And　then　the　maximum　longitudinal　slope　length　under　the　condition　of　different　longitudinal　slope　was　got　at　60　km/h　design　speed.　The　conclusion　can　provide　a　reference　for　the　designing　and　operation　management　of　expressway　long　steep　downgrade　in　mountainous　area.　©　2017,　Editorial　Department　of　Transaction　of　Beijing　Institute　of　Technology.　All　right　reserved.