A　model　has　been　developed　for　the　interaction　between　buoyant　smoke　layer　and　sprinkler　spray.　Based　on　theoretical　analysis,　the　ratio　of　the　maximal　drag　force　of　unit　area　D0　to　the　maximal　buoyancy　force　of　unit　area　B0　in　the　spray　region　is　proposed　as　a　new　criterion　for　predicting　the　stability　of　smoke　layer　under　sprinkler　spray.　To　validate　the　criterion,　sprinkler　pressure,　and　smoke　temperature　and　depth　are　measured　using　an　experimental　system　located　at　the　PolyU/USTC　Atrium　for　sprinkler　spray-smoke　layer　interaction.　D0　and　B0　are　calculated　by　the　3rd-order　Simpson　method.　The　experimental　results　show　that　the　smoke　layer　remains　stable　when　the　D0/B0　is　less　than　1,　and　a　downward　smoke　plume,　which　represents　the　instability　of　the　smoke　layer,　forms　when　the　D0/B0　is　more　than　1.　The　length　of　the　plume,　proportional　to　the　depth　of　the　smoke　layer,　has　an　exponential　growth　relationship　with　the　D0/B0.　The　driving　force　of　the　down-flow　is　the　difference　between　D0　and　B0.