Evaporation　of　water　droplets　while　traveling　in　hot　air　layer　will　be　studied.　The　air-droplet　system　is　analyzed　by　solving　the　mass,　momentum　and　energy　conservation　equations　for　each　phase.　The　droplet　phase　is　described　by　the　Lagrangian　approach.　Two　conditions　of　air　flow　in　the　smoke　layer　are　assumed.　Firstly,　as　commonly　used　in　modeling　fire　suppression　by　water　spray,　the　smoke　layer　is　assumed　to　be　quiescent.　Secondly,　both　gas　cooling　effect　and　air　entrainment　in　the　water　spray　cone　are　included.　The　properties　of　gas　phase　related　to　evaporation　are　specific　heat　capacity,　thermal　conductivity　and　dynamic　viscosity.　All　these　are　evaluated　by　the　one-third　rule.　The　Runge-Kutta　algorithm　is　used　to　solve　the　ordinary　differential　equation　group　for　the　droplet　motion　with　heat　transfer.　Droplet　positions,　velocities,　temperatures　and　diameters　are　calculated　while　traveling　in　the　hot　air　reservoir.　The　effects　of　air　temperature,　water　vapor　mass　fraction,　thickness　of　hot　air　reservoir,　and　initial　diameter　on　the　droplet　behavior　are　analyzed.　The　quantity　of　heat　absorbed　by　a　single　droplet　is　calculated.　Results　are　then　calculated　for　a　water　spray　by　taking　it　has　many　droplets.　The　cooling　effect　of　the　water　vapor　produced　is　considered.　Water　spray　consisting　of　small　droplets　should　absorb　more　heat　while　acting　on　the　hot　air　layer.　The　ratio　of　the　heat　for　vaporization　to　the　total　heat　absorbed　by　water　can　go　up　to　0.9　when　all　the　droplets　are　evaporated.　Limited　experimental　data　are　selected　to　verify　the　mathematical　model.　Predicted　results　are　useful　for　studying　fire　suppression　by　water　mist　system.