A　zone　model　is　proposed　for　studying　the　obstructed　fire　suppression　by　a　total　flooding　water　mist　system　in　a　chamber　with　different　ventilation　conditions.　This　is　a　one-zone　thermal　model　mainly　for　simulating　scenarios　where　oxygen　consumption　and　dilution　by　water　evaporation　are　the　main　fire　extinguishing　mechanisms.　The　water　flow　rate　and　volume　mean　diameter　of　a　droplet　has　been　used　to　characterize　the　water　mist　system.　The　fire　is　taken　as　a　heat　source,　and　its　heat　release　rate　is　related　to　oxygen　consumption.　Mass　and　energy　conservation　equations　for　gaseous　species　in　the　enclosure　under　fire　have　been　solved.　Fire　suppression　is　predicted　by　investigating　the　limit　of　oxygen　concentration.　Numerical　results　are　verified　by　two　sets　of　experiments　on　water　mist　fire　suppression　system　reported　in　the　literature.　The　predicted　results　agreed　qualitatively　with　the　experiments.　In　addition,　the　predicted　results　by　this　model　are　also　compared　with　other　existing　models.　Factors　affecting　the　fire　extinguishment　mechanism　are　analyzed,　and　the　limitations　of　the　model　are　discussed.