Based　on　the　field　survey　of　road　traffic　data,　micro-simulation　software　VISSIM　was　calibrated　and　validated.　Impacts　on　the　capacity　of　exit　were　analyzed,　and　the　velocity　and　volume　of　exit　was　utilized　as　evaluating　indicators　to　study　the　varying　laws　of　extra　lanes　in　the　case　of　different　traffic　flow　combination.　The　outcome　show　that　the　length　of　extra　lane　is　concerned　with　the　traffic　flow　of　the　exit　and　the　secondary　road,　and　the　reasonable　length　is　recommended　as　100~120　m.