This　paper　studies　the　fire　spread　in　horizontal　long　and　narrow　space　with　urban　tunnels　as　the　background.　The　length　of　the　near-fire　source　area　is　a　key　factor　to　study　tunnel　fire　with　hybrid　field-network　simulation　method.　The　length　is　influenced　by　the　temperature　difference　in　the　cross　section　and　the　rate　of　temperature　difference　in　long　and　narrow　space　with　different　tunnel　widths　and　heat　release　rates.　The　theoretical　analysis　and　numerical　simulation　methods　are　used　to　consider　the　influence　of　long　and　narrow　space＇s　width,　heat　release　rate,　effective　ceiling　height　and　the　distance　from　fire　source　on　the　smoke　propagation.　The　results　show　that　the　length　of　the　near-fire　source　area　are　basically　affected　by　the　cross-sectional　area　of　the　long　and　narrow　space　and　the　heat　release　rate.　For　the　type　t2　ultra-fast　fire　model,　when　the　heat　release　rate　is　15MW　and　the　length　of　the　near-fire　source　area　is　115m,　the　smoke　flow　can　be　fully　developed.　Finally,　this　paper　proposes　a　prediction　model　for　estimating　the　length　of　the　near-fire　source　area.　©　2020,　International　Association　of　Engineers.　All　rights　reserved.