By　using　the　piston　effect　generated　by　moving　subway　vehicle　in　tunnel,　the　fresh　air　could　be　introduced　to　halls　and　station　platforms　from　ground　entrances.　Firstly,　the　flow　characteristic　of　the　fresh　air　is　analyzed　theoretically.　Taking　a　certain　station　of　loop　line　2　of　Beijing　subway　as　an　example,　we　make　an　actual　measurement　about　the　flow　state　of　the　fresh　air　at　different　hours　at　the　station.　It　shows　that　under　the　influence　of　the　piston　effect,　about　71400　m3/h　of　the　fresh　air　from　outdoor　environment　can　be　introduced　while　about　54400　m3/h　of　the　dirty　air　can　be　pushed　out　synchronously　when　it　is　not　so　crowded　at　the　station.　And　at　peak　hours,　about　60520　m3/h　of　air　can　flow　in　and　out.　It　means　that　the　piston　effect　plays　an　active　and　effective　role　in　the　natural　ventilation　of　subway　stations.