This　paper　has　analyzed　the　asymmetric　flow　effect　of　fire-induced　thermal　flow　in　a　horizontal　tunnel　under　the　natural　ventilation　condition　by　conducting　large　eddy　simulations　(LES).　The　key　objective　is　to　reveal　and　to　have　a　better　understanding　of　the　asymmetric　flow　effect　caused　by　the　upstream　and　downstream　tunnel　length　difference.　The　mechanism　behind　it　can　be　explained　based　on　the　conservation　of　mass　and　dynamic　force　analysis　on　the　smoke　and　fresh　air.　The　strength　of　the　asymmetric　flow　effect　is　characterized　by　the　mass　flow　rate　of　the　induced　longitudinal　flow　(net　mass　flow　rate　of　a　cross-section).　An　empirical　correlation　to　predict　the　induced　longitudinal　mass　flow　rate　is　proposed.　Furthermore,　the　law　of　smoke　and　air　flow　distribution　within　a　horizontal　tunnel　is　established.　The　proportion　of　smoke　(or　air)　flowing　out　(or　coming　in)　through　the　opening　increase　(or　decrease)　linearly　with　the　increasing　distance　between　that　opening　to　the　fire　location.　The　variation　of　the　air　flow　with　the　longitudinal　fire　location　in　a　tunnel　is　more　sensitive　than　the　smoke　flow.　Results　have　shown　that　as　the　fire　approaches　the　tunnel　exit　from　the　middle　of　the　tunnel,　the　smoke　spilling　out　through　this　opening　is　reduced　from　50%　to　40%,　while　the　fresh　air　incoming　from　this　opening　is　increased　from　50%　to　100%　and　vice　versa.　©　2020,　Tsinghua　University　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.